Good foods in; bad foods out.
Start simple. When kids are young, you have the advantage because they eat at least half their meals at home. If you stock your cupboards and fridge with nutritious foods and toss out the junk, then your whole family will be eating the same healthy foods. Shop for fat-free or reduced-fat dairy (milk, yogurt, and cheese) and lean protein sources (skinless poultry, fish, turkey sausage and bacon, and so on), and have freshly washed vegetables and fruit available at all times.