Always the best sport for overall physical development. And, as the first image shows, you don't have to be in a gym to get some training in!  A talented 8yo:  ...and this one is 9yo - doing iron cross!   And, finally, an important message from the gym:  

Definitely  the  sport for building strength, flexibility, coordination, endurance... and muscle in the right places! ...and, as he's clearly having fun, a little more fun to finish: