AFARR,
I have a CAR15 set up for my 7yr old son. You might have seen it/him in action at the last "pit session" over Memorial Day. It's a 16"HBAR that I had Kurt'sKustom do a M4 job on. The result is a barrel that is lighter than a M4 (smaller O.D. to begin with), and handles super nice. I permanently pinned a telestock to the shortest position to finish it off. Yea I know, there's debate on this, but trust me....you would have to destroy it to get it to move. The result was a super-light carbine that is the proper length for his frame and he enjoys shooting it. When he grows a bit, I'll buy another stock, move it down a pin, and do the same thing. The shorter one will be saved for his younger brother.
A .22 short will kill just as dead as a .223. The key here is maturity, not caliber.
sorry....had to rant!