Eat with enthusiasm to let your children know you enjoy the new food. Talk about the new food while you are eating it. Let your children choose. Take your children to health food store, farmer's market, or vegetable section of the super market. Let them choose what they want to eat.
Many foods have similar nutrients.
Let your children choose their meals at first, even if it means cooking, different meals for different children during the conversion. Children also prefer different brands of products like veggie burgers, soy sausage, and other meat analogs. Indulge their special tastes for a few months, at least. Although your final goal is a diet of whole, unprocessed, and fresh foods, first go all vegetarian. Slowly evolve to whole grains, fresh fruits, new vegetables, legumes, and unprocessed foods.