.243 or 7-08 gets mt vote for a youngster. I favor the .243 though.
A Whitetail wouldnt know the difference but the 7 recoils a little bit more.
My 8 yr old took his first deer this year with my .243. 200 yd shot and dropped it. The kid can shoot, but he's been at it for a while with pellet guns and .22's.