RE: Kid friendly caliber
Real friendly - .243, 6mm Rem, .243 WSSM, .257 Roberts, 6.5 Swede
Fairly friendly - 25/06, .260, 30-30, 7mm/08, 8 X 57 Mauser
Still friendly or able to be made friendly - 35 Remington, 300 Savage, 7 X 57, and more
But anyone of the above calibers can kick your butt in a lightweight rifle with a poorly fitted stock. In a normal weight rifle that is fitted to the shooter properly with a decent recoil pad and proper technique all of the above (and many more) calibers can be tame even at the bench