Well, you have to realize that you can encourage your children or more or less force them. In my experience, encouragement works much better. In a case of unhealthy weight is one situation where I could see not giving you child a choice, but even then, they need to find something that has the possibility of being something at least somewhat enjoyable. Positive reinforcement also works wonders.
My son (25) has done a lot of different things, now he has a fitness center membership and goes 3-4x a week. He went through soccer, gymnastics and martial arts, to name but a few. My daughter (14) went through ballet and gymnastics and now is in her second year (and 4th belt) of Kung Fu.
IMO, kids NEED to be - and stay - active. But you should let them find their preference on how to do this. It can be expensive, going through different phases, buying outfits, paying membership fees etc. but kids cost....lol.
I also do power walking with my daughter. We try to do 2-3 times a week, we go about 45 minutes through a nearby park with hills, and we are pretty sweat drenched when we get home. But the real advantage of this is we talk a lot while walking (in between panting....) and it has become one of our best times together.
