I have no idea why you would need anything more than a .22 for an 8-10 year old, or why you would consider a .22 too boring for that role. Are you wanting your kid to learn gun safety and proper shooting technique the easy, much less expensive way, or is this closer to a "my boy started with a AR! Murica. Fuck yeah." situation?
Ruger 10/22 is the first thing that comes to mind, but the small bolt actions by Crickett, Henry, Marlin and others would probably be better. .22 ammo can be pretty dirty - can't have those kids getting frustrated with jams and lots of cleaning now can we?
As others have mentioned, there is a element of safety in going with, you know, non-military grade arms for inexperienced prepubescent children. Why not get 'kid ammo' for the kids, and save the heavier stuff for when he's more mature and knowledgeable? He will get the basics down in safer, less expensive way, and be more informed and choosey when you take him shopping for AR components so he can build his own!