Depends on the kid... age, size, experience, maturity all that.
.44 Mag
.45-70
7x30
.30-30
.50 cal muzzleloader
.375 jdj
.309 jdj
I'm sure a host of others...
Of course it all boils down to shot placement and the proper bullet. Many will poo-poo the .30-30 but in it's early day it was a powerhouse for a few years. Get close, get accurate, get a good bullet in the boiler room and then the work will begin.
NGH