My 6-year-old son practices taekwon-do. He LOVES his classes and a child can move at their own pace, which is great. It's excellent discipline for him, too. (He's a very energetic kid, to put it mildly.)

He also plays tennis twice a week. I've asked him every season if he'd like to do a different sport, and he says he likes tennis and wants to keep playing. He started off with the Quick Start program, and is in beginner's level now. Same sort of thing as taekwon-do where it moves at the child's own pace.

Good luck!