Childhood obesity is an increasing problem, and the best way to avoid it is to get your child involved in sport and enjoying regular exercise as early as possible, starting right from the play-mat! Also, try not to give them too much of a wide range of foods too soon.
As your children grow up, encourage them to exercise: cycling, swimming and any kind of sport will keep them strong, healthy and stop them putting weight on. Also, give them structured meal-times every day so that they don't snack excessively, and encourage them to chew their food throughly instead of wolfing it down.