RE: Youth Rifle
The Savage 11 also comes in a youth model with a detachable magazine and muzzlebrake. When purchased, Savage includes a coupon for an adult sized stock when you are ready to get one. I got the Savage 11FYCAK in 7-08 for my grandson. This rifle is an excellent shooter and with the muzzle brake is a joy to shoot. As for a shotgun, the Rem 870 youth in 20 ga. is also a fine choice.
Although your thoughts of a .243 / .410 combo will suffice and will be a reliable firearm for your child and the .410 is adequate for squirrels and rabbits and such, he will probably harvest more game with a 20 ga simply because of the increased shot payload. Especially for grouse and pheasants. And too, with the 20, he can also use it for turkey with the proper choke.
Anyway - its always great to see a person getting their child involved in hunting and shooting and getting them outside instead of in front of the boob tube playing videos, etc. Good luck.