Set a good example for your kids. Instead of spending the evening in front of the telvision, find some activity that will keep you moving. In warm weather, bike outdoors; it it's cold, get a stationary bike, a treadmill or a trampoline.
And it is never too late or too early to help your kids build strength and endurance. Push ups and pull ups are an excellent way to build upper body strength. Jungle gyms and monkey bars are freat for kids because they utilize their own body weight. And they are loads of fun.
Keep in mind that kids are not always naturally limber and that their muscles may be tight and vulnerable to injury during the growh spurts that occur during the elementary years. Be sure to include stretching as a part of uour fitness activities.