I built a custom dedicated .22lr using TACCOM3G parts. It's a great rifle. The only reason I didn't buy their complete rifle is because I wanted to use a matching MAG Tactical magnesium upper and the MI keymod rail.... There should be an entire thread in it here somewhere. I gave it to my wife as a valentine's gift.
Honestly, I'd build the kid a regular .223 and then invest in a .22lr conversion. Most of the "dedicated" uppers use the same conversion kit, with some modifications.
Also, invest is a few cans of "Gun Scrubber". I have a few thousand through mine now. That damned .22lr is a dirty little bastard. After a few hundred rounds, I start to get a few FTF's. Just squirt that SOB down, shake the shiit out of it, and get back to shooting.