Ok, I'm a new DM running the Lost Mine of Phandelver for my younger siblings, who are new to the game. Perhaps I'm not the best teacher as I haven't DM'd before and haven't played much, but they're learning quick, so I'm doing something right. My 14 y/o sister is playing a Lvl. 2 Half-Elf Rogue, Criminal background, with the intention of going with the Assassin archetype. My 11 y/o brother is playing a Lvl. 2 Variant Human Fighter with the Athlete starting feat, Outlander background, with the intention of going with the Champion archetype. He also wants to be Wolverine eventually, so I'm working on making some magic items to give him later to fit the theme he wants. I'm playing a Lvl. 2 Hill Dwarf Life Cleric, mostly just to ensure that the others don't die. And I kinda want to play too, and we needed more people anyhow.

As soon as we got to Phandalin, Maki (the Rogue) threatened a guard, two civilians, and the mayor, got in a chase, hid in a tree until the next morning, and proceeded to burn down the mayor's house, all because of the mayor being too busy trying to figure out how to get rid of the Redbrands to address her as soon as she walked in. Notably, nobody knows she did the arson. There are suspicions though. On the other hand, Victor (the Fighter) walked to the inn to check on his Golden Retriever he mentioned in his backstory, Bailey. He's a guard dog that Victor had crafted rudimentary armor for. There, Victor learned that the dog had fought off some Redbrands that attempted to mug the innkeeper, and proceeded to chase the Redbrands out of town, to the south. I thought that Victor would gather the party and then search southwards for Bailey. Nope, he went alone. He would've had to go through the whole backup Redbrand hideout alone if not for, by dumb luck, rammed into Beorn (my dwarf) at the woodcutter's and dragged him along. I set this up as a loose thread to pursue later, after the main Redbrand base was cleared out, but no. Also, Maki has no clue this happened. She's been looking for Victor and Beorn for a full day, but her character doesn't we're not there; bad Intelligence check rolls.

Now, as DM, what do I do? Victor's been going full John Wick on these poor Redbrands, slaughtering Hard and Lethal encounters with near flawless dice rolls. He ain't cheating, he's just lucky and his foes keep getting 4s and 5s. Maki is now wanted in Phandalin and Neverwinter (acted as hitman in Neverwinter in backstory). She's not doing well on the public scene. My cleric is just watching this havoc unfold. Please help.

You know not everyone will tell you this because its "unfashionable" to say it but there is such a thing as "skill" at role-playing and I don't mean acting in character, but the very tangible skill of knowing how to succeed at the game through a sort meta manipulation and logical solution to problems.  Essentially thinking about possible outcomes to the actions you're considering and realizing/knowing what is and isn't a good idea.

Now adults tend to "see" that part of the game much faster than kids and teens.

The best way to teach them is to show them that this imagined world is a real place in the context of the story with real consequences.  Don't stop them from doing anything, simply ensure the world responds in a believable and natural way.  If a player starts a fire, there will be an investigation.  Did the player leave any clues?  where are their witnesses?  

The other thing to keep in mind is that the adventure you are running will be about whatever the players not you decide it's going to be about.  You thought it was going to be about them solving Lost mines of phandelver, but it may in fact be about the story of a pair of outlaws running from the law.  Both can make for interesting stories, but you have to be able to adapt and adjust your plans.  If your unwilling to do that, if running the story you prepared is paramount to you... then you need to stop the game and talk to the players about it and remind them that "we are playing the lost mines of phandelver, there is purpose and intent in the game and they have to go along with it".

I recommend going with the flow and see what happens, but those are basically the two options.  Play it out and see what happens or stop the game and have a talk.

There are points in which you can and should pause play. Especially with new players. Somebody running off on their own is generally a bad idea. Pause the game and ask the ubiquitous "Are you sure you want to do that?"

Experienced players know this is the GM giving a hint that what you are about to do is foolish.

For the fire thing, you're the GM. Just make it so that somebody saw it happen. Maybe the Red Brands saw it? Maybe some street kid saw it? 

You control how tough the bad guys are. It is completely ok to give them more hp or higher damage bonuses. Of course it is also ok to let the player have his fun. 

If the party is split and you want them together, just put them together. It isn't fair to the rest of the players if somebody does a lot of solo activity. 

A little unorthodox, but get new dice.

The player characters get arrested, the adventure is over. 

That's how I'd handle it. It sets the tone that if the players want an adventure you can help provide that. If they want murder-hobo lifestyle, actions ahve consequences. Thieves and murderers do not become the heroes of the realm.

That here is the issue of course. D&D tend to attract players who don't think there will be consequences for the actions of their characters. There can and should be. It helps minimise toxic player actions and helps keep some kind of boundries on the adventure and action. 

I had a kinda similar situation a while back DMing a Curse of Strahd campaign, though in that case all the players were veterans and knew what they were doing. It was therefore a little harder to get them together, since they knew exactly what they wanted. But if I gave them a little nudge, they tended to start thinking, hm, I wonder what the benefits would be to doing this the other way. for example, when they sent a PC off on a solo recon mission, it went almost exactly to plan. except, he ended up wandering a little to far into the fog, and was later found by the rest of the party on the ground, knocked out and robed. They only sent someone on a solo mission one time after that, and it was a very short thing.
When the campaign was over, we had our typical talks about what would've happened if they had done this or that when, or why I did a certain thing, and we started talking about when I did this.
I told them that I knew he would not only die if he went of on his own, but also, it would kinda ruin the story in this specific scenario.

Anyway, I guess the gist of what I'm saying, is don't be afraid to hinder Victor in a way that he knows he's going to need to go back and get help. And like the others said, it's perfectly fine if Maki is arrested for their actions. They did, after all attack several people and burn down the mayors place.

I played in a campaign a while back where I was playing a criminal, and it happened several times where I failed to cover my tracks, or picked on the wrong people, and had to face the consequences. That's part of playing D&D. You can't just get away with whatever you want, even though that seems to happens a lot.

I'm not going to say you need to make consequences happen to teach them a lesson. That can easily lead to escalation, especially with young players.

What you need to do is get your players to agree to the expectations of the game up-front, before they even make characters. In this case, those are "You lot are a team. You know each other and work together. You are the good guys, and you are here to do this adventure." (Bad characters can be fun and interesting to play, but for a first-time player with a first-time GM and a premade adventure, it's going to go badly.)

I don't actually have any idea how to fix the game you're currently running. The best way to get the party together is just to get them together. Don't use skill rolls if failure means the game doesn't happen. If you want her to learn where the other PCs are, tell her. Make it possible.

I might try having the rogue get caught, and have them offer to suspend her sentence if she agrees to help with the Redbrand problem, and coincidentally there are some heroes who are doing good work there, so they're going to bring her to assist them. Then just cut to the guards catching up to the other PCs, and hand her over.

That sorts the split party through heavy-handed use of DM power (a thing to be avoided in general), but not the underlying issue with the character/player being somebody who'll go off on the NPCs for no good reason. That's only fixable with an out-of-game conversation. Pointing out that a hitman on the run ought to be used to subtlety and shouldn't want to draw attention to themself might help, but I wouldn't bet on it. Swapping in a new character that's a better fit might be easier, but it requires the player to want it.

Random encounters can be your friend. Have a group of bandits blocking the road or whatever. Just need enough to make him reconsider traveling by himself.

Of course given his age it's probably more likely he didn't even think to retrieve the other player and potentially didn't have a clue where to look for her at.

In session zero I explained to my group of new players that they are choosing to play a team-based game and that I am not prepared to run separate games for each person. I explained that while they have a lot of creative freedom, they still need to take into account the fact that the rest of the group needs to want to have them around. If they insist on doing things that cause friction or trouble in this living and consequence filled world, it may very well be that their character won’t be welcomed to adventure anymore and the game will be effectively over for that character. As others suggested, you may need to clarify if they are actually interested in solving the issue of the mines and the Black spider and if you’re interested in just running a straight homebrew in that setting. That’s an out of character talk to have together. Getting them on track may be as simple as having Black Spider or his minions ( like Glasstaff) go after the whole party in some fashion. Make it personal…the bbeg is not just a problem for townsfolk they just met and may not care about, but is a problem for them specifically.

Finally, if you’re playing a regular NPC working with the party, keep your role simple and make sure you are never part of the decision making process. As DM, you know too much to ever do that. Consider using a sidekick template or Matt Coleville’s Followers template so you have less details to worry about. You’re already playing every other person and monster in the game.

Maybe try some less story heavy adventures with them so they can have their fantasy of kicking ass and taking names outside of a situation that messes up your game. 

Tales from the Yawning Portal has a lot of good adventures for that, lots of good funhouse dungeons that younger players really enjoy. 

I hesitate to advise you to show real consequences  for the players' actions because, while that's generally great advice for adults, when I was younger I definitely vibed with the fantasy of being able to do anything whenever, and that kind of freedom that no other games let you have is a big part of the draw for younger players. 

I think better advice would be to run the game long enough for your players to genuinely become invested in their characters and your world, and *then* try to bring them into a big narrative driven campaign. 

Quote from martintheactor >>

The player characters get arrested, the adventure is over. 

 This is very poor advice for new players that are also young.

At the very least have an out of game talk with the players noting that running off on their own is a bad idea as well as causing unwarranted harm to the townsfolk. The town needs their help and have already been abused. For the rogue, it could be a redemption story. 

I'd also try to steer them away from assassin. It's poop. Even the 2024 version is poop.

Reading your post, the issue (you as the DM have created) is the players believe and are acting as there are no consequences.  If we are being honest, most DMs are incapable of separating themselves from the players (either FOR or AGAINST) and just DM the game/world as they should.  The DM should never side "with" or "against" the players, and allow the players actions to play out, allow the dice to play out as rolled (stop faking rolls).  This means you play the NPCs as smart, logical entities.  This is the one thing I have found the vast majority of DMs are utterly incapable of doing.  The monsters use their abilities (ALL of them) fight as a team to actually win,  the NPC play like characters in the aspect of logic and reason. Don't fight the monsters or the encounter so the players "will win." If you are doing this, your encounters are designed wrong. 

What would happen if someone burned down a house in your actual (RL) town?  Would the Mayor, police, etc., simply ignore it and go, hey I think they did it, but we will simply ignore it.  Absolutely NOT.  This is what I mean by the DM not running the game correctly because they want to "protect" the players and the game.  Over 35 years of DMing and thousands of hours of playing, this will ruin a game faster than a TPK.  Players die, if they never die in your game then you are simply not running the game as intended.  No risk, why play?  This is why a lot of parties turn into murder-hobos,  players lose interest, stop showing up and the game fizzles.  Crappy DMs.  

This also should have been addressed in session ZERO.  Where a DM goes over how the world will play, explain the monsters, NPCs will act intelligently, monsters will fight in accordance with their intelligence and party/player death is a potential outcome.  You obviously gave the players the idea they are invincible, there are zero consequences for their actions, and in the future things will go even further off the rails.  

Your players are young yes, but not children stop coddling them.  Get ahold of your campaign now or don't be upset when it burns down. 

Update: Rogue player was angry with me for... telling her I wouldn't stop the Fighter player from responding with deadly force if she attacked him. So she quit. Whatever though, bullet dodged probably. In the meantime it's just me and my brother. 

In order to make up for the fact that there's now only one player and my personal PC, I used the sidekick rule from Tasha's to give the Fighter's Golden Retriever a stat block. I'm ruling that he can expand a bonus action to give the dog commands in combat, and if he doesn't, the dog will follow the last command given (or if impossible, will defend the Fighter). Does this sound OK?

Quote from TherenTheLost >>

Update: Rogue player was angry with me for... telling her I wouldn't stop the Fighter player from responding with deadly force if she attacked him. So she quit. Whatever though, bullet dodged probably. In the meantime it's just me and my brother. 

In order to make up for the fact that there's now only one player and my personal PC, I used the sidekick rule from Tasha's to give the Fighter's Golden Retriever a stat block. I'm ruling that he can expand a bonus action to give the dog commands in combat, and if he doesn't, the dog will follow the last command given (or if impossible, will defend the Fighter). Does this sound OK?

 Yeah, not bad.