.243 or 7mm-08
I started my son with a .243 when he was 10 (because i happen to have two on hand)and he is very proficient with it now at age 12. I have personally killed 23 deer with a .243 rem 700 . the 700 has a little less recoil than the youth modle seven because of the longer barrel.with a good rest she should be able to shoot very well .