Pairing a lvl 20 with a couple of lowbie ?

Hi guys, has any of you ever had succes allowing a really high lvl to play with new characters ?

My kids play and one of them would like to keep playing his OP toon while the 3 others want to start fresg.

That sounds like it could be really frustrating for the ones who are starting over, particularly if this is an overpowered 20th level character. I won’t say it objectively can’t work out, but you need to be really sure the player for the 20th level character can step back and not steal the show and/or the rest are okay with having someone in the party who can effortlessly melt enemies that would be bosses for them.

Rightfully so, you don't give the kid's ages.   If you have one child that is able to play a 20 level character, then they should be mature enough to set that character aside and be the same level as the others.

If they can not set that 20th level character aside, and start fresh, what does that say about how they will play with the others?

If anything getting the child to set aside his much adored 20th level character is a lesson in sharing and maturity.

Ignoring all questions of intent and maturity, a level 20 character can't meaningfully play with low-level characters. If they participate, they become dominant, both in combat and non-combat situations. Their skills are significantly higher. Any fight that challenges the low-level characters will be annihilated by the L20. Any fight that can credibly threaten the L20 can instakill the low-levels.

Yeah, thatd be tough.  A level 20 anything is crazy powerful compared to a level 1 anything. It would be really hard to keep things fair for all the players and hard for the dm to not accidentally kill everyone except the level 20 character.

At level 1, you can die permanently just by falling off a fence. At level 20, you can get hit by a disintegrate ray and probably survive.

Ive had players get up to level 20 with a charavter they love, and when we did a new campaign, their old character became an npc they got to speak for that worked in one of their bastion facilities.  It kinda lets them revisit and reminisc about thr character, without unbalancing the game.  

The issue you'll have is balance.

I've seen a TPK from an Animated Armor at low level. They're really weak, and it doesn't take much to put a level 1 character down - a lucky damage roll will do it. A crit isn't something that hurts, it's something that kills, even from the standard fare for low levels. Meanwhile, a L20 won't even notice that they're being attacked. That Animated Armor that has the potential to TPK a L1 party? It averages at 11 damage per Round (assuming no Attacks of Opportunities). The weakest Class in terms of HP (either a Sorcerer or a Wizard) at L20 will average about 70HP and can kill dozens to hundreds (depending on how they're spread out) of Animated Armors in a single attack, no problem.

So how do you make a fun fight for them? Anything that can make a level 20 even care would kill the level 1s just by looking at them. Yes, a Tarrasque would probably make for a very interesting fight for your L20, but the L1s might as well watch the D&D film instead. Social and other kinds of encounters are little better. Your level 1s will probably have their best Skills at +5. The level 20 will have a lot of skills at +11 or higher - plus several abilities to help it even more. Why would the level 1s do anything?

I really wouldn't mix these groups. Either everyone carries on with their L20 characters (use Epic Boons if progression is the concern?), or everyone reverts back to L1. D&D just has too steep a progression for this to be fun for everyone. Even with a couple of levels, the difference becomes noticeable (but manageable), but such a wide gap, especially with level 1 characters? That's not going to be something that you'll overcome with everyone happy.

Whatever you choose, it is important to have everyone at least close. I have played at an XP table where small level differences were okay (but definitely noticed), but I can't imagine it being very fun at all for there to be large gaps in levels.

If the child really likes the character you could have it suffer some kind of level drain, commit to a new class and forego everything from the old, etc... This would let them RP a beloved character in a new way without totally unbalancing the game. What you stated makes me think the child still wants to be able to cast Wish, make 4 attacks a round, or whatever is appropriate for their level 20's class. I see no way that is going to be fun for children (or the vast majority of adults) with a huge power disparity.

Sometimes, a DM has to put their foot down and say no for the good of the game. This is one of those times. Tell the kid that there is the possibility for other games with their level 20 character, but they can't remain at a level 20 power level and play with newbies. They have to start out at the same level as everyone else.