There is a big difference in recoil between a semi-auto .223 Rem and a .243 Win bolt action. I wouldn't let most under-10 kids shoot the .243 Win, unless they're big/tough for their age and have had adequate shooting experience.
I've seen 10 year-olds handle one or two shots with a .243, but subsequent shots at targets created flinches that may last quite a while. One pulled his head away from the stock, just before "jerking" the trigger.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.