It's not always easy to figure out how to keep our children fit. There are a lot of distractions. There is also the problem of schools and the food served there. Sometimes that is the place to start.
Investigate School Food: Parents are usually allowed to eat with their children, especially if they are volunteering in classroom time. Eating with them a few times will let you see what they are serving. Look for these things:
Is it healthy or is there a tablespoon of grease in the middle of the pizza? Does it look appetizing? Does it taste good? Are the kids eating it or throwing it away?
Work With the School: After finding out what's being served you can address any problems you see. The school district may be able to help you create better food from the cafeteria. They may also suggest you pack your child's lunch if the food isn't acceptable. The former is preferable but if the latter is the attitude packing lunches isn't that difficult.
Encourage Play: You can do this both at school and at home. I was allowed to be on the play yard with our children after I signed in at the front office. This is probably still true. It not only helps the person in charge of the playground it encourages the children to play.
Join In: If at all possible, play with them. At school they may be doing team games, or just throwing the ball around. Playing with the kids is fun, it's good exercise for you and it shows all of the children that this is something adults do as well as children.