The best bullet for a deer in 223 is .243
Leave the 223 in the closet for deer
Get a Tikka 243 for the youngun and never look back
in this day an age, get them an arc or a Grendel.
they dont need all that extra muzzle flash - even though the recoil is relatively mild, at least compared to its bigger brothers.
either of those calibers will get the job done, with less felt recoil, and less flash than a 243 will.
likewise going with a 350L.
im not saying theres anything wrong with the trusty ole 243 - its a proven deer anchor - just that there's way more options these days for youth hunters to start with on a small frame low recoil deer rifle that they could also conceivably hunt their whole lives with.