So this is mostly a balance issue.  I am a leader at my church's youth group and we are having a lock in this weekend.  Several of my guys and a few fellow leaders expressed interest in playing D&D.  I don't have time to create a story and had the Tyranny of Dragons campaign(Hoard of the Dragon Queen + Rise of Tiamat).  I wanted to do something that had the possibility of turning into a campaign as some have said they may want to continue play, but could stand if we end up just having 6 hours of play during the lock in down time that could be a fun intro for a bunch of guys to table top role playing games. 

Anyway reading the campaign, I see that first section is meant to be a single night and requires fantastic resource management from a first level party.  As this group of 8 are almost all no experience, 2 who've played 1 session, and 1 kid whose played maybe 10 times in his life; I don't think I want to brutalize them with that.  I think though turning that slog into a week long siege would work with a few tweaks aside from adding what I need to for a large party.  My question though is that since they are resting and if they get enough xp to potentially level mid session will this throw off the campaign, especially if it means someone can actually survive and win that final fight? 

1.  You don't have to worry about them winning the final fight even if they are level 2.  If you are really concerned about it, well that dragon doesn't have to leave...

2.  Throw them into the fire, if they burn all their resources, well, give them some "plot armour" to get them through it, barely...

If you are really worried about it, switch to LMoP, 6 hours will get them through the first act, get a level-up, and get involved into the "meat" of whichever quest line they choose.

Or if you don't mind a little conversion or searching, B2, Keep on the Borderlands has been a staple for years, Into the Borderlands is the 5e version...

Edit: sorry,  the below may not apply at all if you are set on Tyranny of dragons.

If you have access,  what about the intro module,  Lost Mine of Phandlelver.

Start them at level 3 in Phandolin and offer redbrands, cragmaw and one of those could lead to the lost mine.

I ran through that first part with some new players. You don’t have to use every encounter, feel free to skip a few if they are running low. Throw in a few extra health potions and it can fix a lot of hit point issues, at least. And just use milestone leveling and you don’t have to worry about them going up too fast. 

As said above, you don't have to use all the encounters.  You can pretty much make any combination of enemies you need as the players move through town.  Once they are at the keep you can get away with one short rest in the night and there can be healers or potions anytime they return.  

I´m atm running Hoard of the Dragon Queen as a campaign.

The group size is nearly the same with 7 players. I run the first chapter as one night but with a few tweaks to adjust it. Give the group some reasons to explore and help Greenest,which is covered in smoke and flames. The Lord is known to be very protective to his people and would use the few ressources to get a little bit more help.

I would also give the group the information that it will be a long night.It´s a dangerous place and every shadow in the smoke can be someone who needs help ,a friendly face or a foe , so you can sneak very good in the first hours. If you read it a little bit further,you can already drop small hints for the next chapter f.e some chatter about their commanders like Langedrosa.

For the resting, i used a additional feature - a fairy tree that grows under the castle in a small cave,connected with the fort. It´s a old trade with the Lord and the local fairy´s and if the PC showed themself as protectors, they agree to give them some energy → this can be a full recovery ( a long rest) or some part of it and it can only be used once from a PC. It´s up to you as a DM to decide.

Things like Health Potions or similiar should be given only a few. They still should feel that this is a emergency and they ran unplannend into and even Greenest is not prepared and there is no shop or similiar open. A part of my group traveled with a dwarf merchant ,who had some working tools,a few trade goods and a few health potions that went later on also into the fort and is the improvised blacksmith in the fortress,which can be used as a little guide for other things.

1. Not concerned in the sense of being heart broken at the idea of one of them winning.  More I just wanted to know will it hurt the future plot if that guy doesn't survive as the campaign seems to lean heavily into the idea that he will.  If I read that part right I don't think the dragon born commander would allow that interference.

2. I understand the idea and if the were at all experienced players then sure, but these are for the most part kids who have never played before.  They are going to be just learning the game and likely having fun using all their shiny new powers.  I don't want to curb that enthusiasm in new players just deciding if they like D&D.  Also I have had a lot of fun playing through scenarios where my character is pushed to the brink of their resources, but from having played with and run games for new players I have never seen it be an enjoyable thing for new players.  They are just learning to use their characters and just figuring out what they are capable of doing.  I am not saying they shouldn't learn and a couple encounters per rest is normal and means that a person may have to use lesser powers for a combat, boring; but if they are small/short encounters that isn't too  much for a young/new players.  A meat grinder though designed to chew through those resources will leave multiple characters going through several encounters with minimal resources.  That will feel like a slog, and likely encourage the idea that D&D is boring not teach resource management to people who haven't already come to know that D&D is fun. 

I appreciate the suggestions, but have already used a decent enough amount of my prep time drawing battle maps and I promised dragons    

Fair, I may consider it, but this seemed a better option for if time became an issue in real life rather than in game.  I like the encounters there as they feel heroic and show off a lot of different potential sides  and tactics w/in D&D.  Food for thought though.  I know a lot of people like milestone, but I personally still like XP, and if this does become a campaign I want to go on as I started.  

Fair on the use of encounters, though I would like to use them if possible.  If you have have run this though before do you feel that simply extending the time, with the possible side effects I have listed or others you are aware of would hurt the campaign?

My issue is that only fixes half the resource issue.  Yes they can then potentially keep going, but for new players I think that going through multiple combat scenarios without access to their magic or powers besides cantrips won'tbe fun for the new/young players I am introducing to D&D for the first time.   

Ok thanks I like the idea of a fairy ring.  It'll even let me throw in a little lore on time flowing different in the feywilde.  Hmm, since it is intervening magic and not just the normal flow of time I don't want it to be just there at a whim, but could see letting them use it more than once.  What do you think of the idea of once used the connection wains until the next sunrise in the feywilde and since that can be variable, they won't know for sure if it will be available again that same night in the keep?

Since it sounds like you ran the whole thing for your players, did you use XP or milestone, and if XP had they leveled beyond 1st before that final one on one fight?

If you are farther into the campaign If someone does kill that commander would it mess things up?  I have only read through the approach to the camp.

I suppose that it depends on the mix of classes in your party.  A short rest and some healing seemed to work (sorcerer, paladin, cleric, monk, rogue) for my players.  I think you could extend it out to a 2 day siege?  Or, if the characters arrive late afternoon\evening, run an event or two then need to settle in for the long rest you could have the the Half-dragon or Rezmir come up and say you have until sundown tomorrow to surrender.  Squeeze in a long rest, run some of the events during the day (Sally Port Door, Dragon Attack, maybe one sneak out).

Could work.