they make the AR10 in 7mm-08, which would make a GREAT youth caliber.. Also if you installed a quality collapsible stock it would make it the length of pull adjustable as they grow.. They also have the AR10 in 308 and 243, plus a couple others - but i would go with the 7mm-08
If you wanted to use the AR15 lower, which would probably be a tad cheaper and lighter, you can get the 6.5 Grendal, 6.8 SPC, 7.62x39 uppers (plus a few others).. Shot placement would be much important with these..
You could also go with the big bore ar15's - 50 beowulf, 458 Socom, and 450 Bushmaster.. they kick about like a 20 gauge shotgun but are still quuite managable. I have the 450 Bushmaster and its quite accurace..
Brian