Seems like most everyone is telling you to get a 223, what else do you want? It can be very accurate, very low recoil for a centerfire, cheapest to shoot. Great varmint caliber if your into that, even "legal" for deer in some states.
I'm a Remington guy myself, but for what your asking for, I'd actually get a Savage, best bang for the buck IMHO.
If you want (a little) less recoil and are willing to spend the money, get a semi-auto like the AR-15 or Mini-14. They're lots of fun and very expandable if you get more serious.
I started shooting a 223 around 11 or 12, and I was a small kid, so it won't be a problem for your youngster.