I am afraid the 270 Win may be a bit much in the recoil dept for an 11 year old unless you plan on either using managed recoil loads, or reloading your own reduced loads. You might want to look into the 7mm-08, which give 270 like performance with less powder and recoil and it is a short action cartridge. The 260 Rem and 243 Win would also fit the bill.