It doesn't matter. Personally, I'd prefer a .223 with a good bullet (my favorite being 65 grain SGK.)
The most important thing is to get the kid shooting regularly on a life-size deer target so he gets used to putting the bullet where it belongs. Also, that gives you an idea of how far away you should let him shoot at a deer.
The example in the OP wasn't a failure due to using a .223, it was a bad hit...period. The longest goat rodeo I ever was involved with was when a friend's kid shot a small buck badly with a .30-06. Almost no blood trail and we eventually got the deer only because I had a pretty good idea where it was going. The lack of a blood trail wasn't because of "no pass through" it was because it was hit too high in a non-vital area. The best first deer experience I was ever party to was another friend's 12 year old daughter. I set her up with an original 92 Winchester SRC in .44-40 with mild 200 grain cast RNFP's that were barely moving. I don't remember the specific load, but they were only going 1000 or 1100 fps. She shot her deer at about 50 yards (with the original open sights on the 92. The deer took about two steps and dropped dead.
Don't overthink it.
It doesn't matter what the kid uses, just make sure he can shoot it well and knows where to put the bullet on a deer.
