Be an example. Do you exercise, eat right, live a healthy life-style? Leave fresh fruit like apples on your table in a basket. Have string cheese or cut up celery in your fridge for a quick a splash of peanut butter or treats like 'ants on a bridge' (raisins, peanut butter, celery).
No matter what it takes, get your child in after-school activities. My children tired of Karate, and chose jump-rope and cheer this year. They can choose but have to stick to it and the activities this year are much less expensive than the Karate. We got a jump rope from the toy store for less than 5 dollars and she uses it to practice and have fun in our yard on the days without class, already dropping the little weight I was worrying needlessly about.
Whatever you do, don't nag! Don't name-call! Never, ever belittle your child or refuse simple requests like a flowery outfit because they will "look fatter." All these are hidden messages destroying your child's self-esteem and forever labeling them a fat failure in their own mind. Our society is already shaping these patterns despite our best efforts to keep our kids positive. Love your child and do your best to affirm their existence as a force in your family and community. If all else fails, head over to weight watcher or some other evidence-based weight program with successful results from balanced nutrition and activity. Lots of tips on Dr. Oz' Move it and Lose it program!