I personally don't have children, but watching my niece grow up has been a joy. She just turned 10, and I did live with her for the first 5 years of her life.
She has always been an active kid, and I attribute that to 1. her wanting to do various activities, and 2. my parents who are raising her, they were conditioned by me <as a kid and teen> into running around constantly taking me to various practices and activities!
She's a normal kid, she eats well <my mother loves to cook, my father is diabetic and has health issues of his own> and my mother cooks "lite" meals full of organic foods, non-processed, not overly salted or sugared, and lots of veggies. She eats cookies, brownies, and whatever other junk you can think of, but the key is "moderation"....
She also only drinks water. My mother started giving her water at an early age instead of fruit <ie sugar> drinks. Now all she likes to drink is water. We'd offer her anything, from pop to gatorade, to fruit drinks, but she prefers water.... she's 10!!! How awesome is that???
She's also very active. She's tried every sport I can think of, loves horseback riding <she just finished a week long camp that she goes to every year for riding>, has wall climbed, loves skiing, and is a member of a youth group called "indian maidens" which keeps her busy.
Basically I think parents are key in a childs health and well being. Too many parents let their children become sedentary because they themselves don't want the hassle of managing a childs activities. It's sad, but these days video games and computers have become babysitters instead of evening down-time activities... Don't get me wrong, my niece has a Wii.... I'm an avid gamer myself, but again, the key is "moderation"!