As a parent, you need to be in charge, but instead of treating your kids like the peasants in your domain, recognize that they are citizens with some rights. One day they will need to stand on their own and make healthful decisions about their diet, and they won't be prepared to do so unless you start teaching them to make good choices now.
A good way to teach them about food choices is to sit down with them and plan a menu a week, two weeks, or even a month in advance. Make a game out of creating a grocery list for your planned dishes. Take your kids to the store with you and let them find ingredients and mark them off on the shopping list. Remember, both now and when they are on their own, your kids are more likely to eat healthy foods if you involve them in the decision making process by presenting them with healthy choices and letting them take it from there.