7-08 or 7x57 get my votes. I have and enjoy a browning BLR 7-08. Sticking with a bolt action or single shot might be a better idea. I'd look into the Ruger #1 single shots in said calibers. 30/30, 35 remy, and 243 are all good cals with little recoil, but a 7x57 or 7-08 will give them the option of using it for bigger game when they mature. My first rifle was a 30-06 at about 12 years old. If you teach them how to shoot properly, children can learn to handle the recoil of a 308 or 30-06 after they get used to the muzzle blast.