I’m honestly not sure how to do it once they’ve gotten used to a diet of fast food. My family starts kids on healthy food from the beginning, with fast food kept as a special occasion thing. If they’ve never had anything other than good food, they don’t know any different.
I do know Kraft (and probably others) is making mac & cheese with veggie pasta, which can help sneak nutrition into them. And there are a bunch of cake, cookie, and brownie recipes using black beans or zucchini or carrots or a combination of them so it tastes like dessert but is actually healthy. Maybe try making burgers at home instead of hitting McD’s, cutting your own potatoes to make baked fries, etc.?