Having kids that love sport or exercising can make life a lot easier when it comes to getting them moving.  However that doesn't mean to say you shouldn't have concerns.  I loved sport as a kid, and did a few hours every day - squash, hockey, riding, running etc.  In fact it was a running joke that I never wore my school uniform at school - just my sports kit.  However, if I have known then what I know now, I would have gone about it all a little differently.