The transition is much smoother when mom AND dad agree on the diet change. Still the rule is the less fanfare, the better. Slowly introduce more nutritious menus and set a good example of eating and enjoying. Don't neglect an opportunity to point out to the children when a slip away from healthful eating produces direct negative physical discomfort. That way your kids won't be tempted into eating something they shouldn't.
Children learn by example.
A household that manages food properly will help children adopt healthy eating habits. Praise your child's efforts to make better dietary choices. Stock the pantry with healthy foods. Provide balanced meals at regular times. Lead by example.