It's a good idea for school-age children to do some kind of aerobic exercise two or three times a week, for 20 to 30 minutes at a time. As your muscles get stronger, you can do more active things for longer periods of time. It builds flexibility.
Most kids are pretty flexible to begin with, meaning they can stretch their bodies with no problem.
But as you get older, your body is less limber. This is why it's important for kids to exercise - to stay flexible. Exercise also helps kids maintain a body weight that is right for them by burning up extra calories. When you exercise, your body uses that extra fuel to keep you going strong. It makes you feel good.