I guess it's been said already. But a youth with a .223 could possibly be a bad thing in a deer that gets wounded and runs off with little blood trail.
And if you actually find a .223 round with a bullet that is "heavy" enough, your rifle may not have a fast enough twist to be accurate. Heaviest I've seen are 90 gr HPBT match bullets. And those ain't good for a big wound.
I have killed a few deer with .223's before, but they were standing still or laying down and I had a sub-MOA varmint rifle on a steady bipod and made solid forehead shots. Those were instant drops.
Can you get the young hunter to use a larger caliber? Say a .243 Win? There is hardly any recoil to that.
In my opinion only,
MG
PS: Thank you for supporting the youth hunt. It is a fine thing you're doing.