I have had much experience tracking deer from youth shooters using .223 and even some using .243.YES,they are capable of killing deer and I have killed them myself with a .223 with hollowpoints just to check the effectiveness of them and I killed a doe and it went down quickly.The issue I saw was the blood was non existant and tracking is tough.My daughter shot a nice buck and caught bone and the hollow point expanded and we got plenty of blood but the we pushed the deer and lost it.
In short,if you decide to go with it,trweat it like a bow.Even on your wait time in stand.
BUT,imo,I would recommend going with a .308 ,yes I know what it sounds like but after my experience tracking deer for 4 different youths using the .223 I advise the .308 with light loads like a 125 grain or 150 grain bullet.
My son started using his when he was 8 and we have yet to have a deer run off.Rossi makes a youth model and is an excellent choice combined with a red dot scope,the child will have a blast.
This is the gun he used for these 2 kills and both dropped in their tracks.