My experience with several kids has convinced me of the exact opposite conclusion. For a kiddo that's relatively new to shooting and hunting, I prefer a .223 in their hands.
I've seen a handful of kids with limited exposure, and ample time behind a trigger intimidated by a .243
Put a kid behind a .22lr and a .223 and they'll gladly shoot till there's no more ammunition. They learn good shooting habits (and safety of course) and have absolutely no fear in the rifles muzzle blast and recoil (which is almost non-existent). With the amount of time yanking triggers, it's no biggie for them to make clean ethical kills with a .223 loaded with an appropriate bullet.
My own kids, nephews, and friends kiddo's have over the years killed piles of deer, exotics, and feral hogs with the lowly .223 without an issue.
Another one of my nephews and a friends son are just now cutting their shooting/hunting teeth on a .223 Rem I set up specifically for youngsters. This weekend saw them popping 62gr. TSX and even .22lr out to 200 yards from sticks and hunting positions.
Their ear to ear grin is down right infectious. No doubt both will have no issue killing a deer or three this coming Fall.
