There are a growing number of people concerned about getting their kids to remain on the skinny side rather than getting obese. Many of these people are of course obese themselves, so my first step would be to tell you to take care of yourself. Start eating healthier, cut the fried and other fatty foods out of your own diet, and by proxy, you will naturally cut all of those foods out of your kids’ diets. You will also in the long run be teaching them better eating habits in general.
You should also get them involved in sports programs. I see kids at the gym sometimes with their parents, and their parents are telling them how to stay lean and how to get the best workout. They’re 5-8 years old, they don’t care. Don’t focus so much on what it takes to be lean, focus on your kids having fun. Maybe they want to sign up for soccer, maybe it’s football or basketball, maybe they want to go do a little cheerleading camp or dance camp.
Whatever it is, get them involved in sports, get them really going, and you will see the exercise come naturally as it should. Never place a lot of focus on the idea that they are dieting. Just teach through experience.