I put my 11 year old daughter and her 10 year old boy cousin in a CVA hunter .243 single shot when they started hunting, and it kicked the crap out of them. I actually became concerned about concussions after watching them shoot a few rounds, so I backed off to 80 grain bullets and had them practice on my .22 until they actually hunted. People always point to .243's as the perfect "kid's" gun but I think with today's rifles being a full 30% lighter than when that "rule of thumb" came about, we need to think about that recommendation a little more.
That lightweight .243 really turned my daughter off of shooting rifles, and she was a 4-H marksman for years before that (in the .22 program) and loved shooting. If I had it to do over again, she would have been using my Howa Mini 7.62x39 instead, since I wasn't ever going to let her take a shot beyond 100 yards anyway.
Just my observations from a small sample size.