We got our grandson to raise when he was 3 years old, and he had never sat down at a table to eat. His meals had consisted of cold hot dogs, cold green beans in a cup, bread & butter, and blue box mac & cheese.
He was big into the "ninja" thing, so I got him to eat rice dishes with assorted vegies and meats by calling it "Ninja Food". I showed him the Roman Soldier on the Roman Meal bag, and got him to eat "Soldier Bread" instead of white bread. Now a teenager, he eats all kinds of things other kids wouldn't touch.
My smallest grandson, 8, comes here after school a couple of days a week. I let him "shop around" for a snack, and often as not he chooses a healthy one. Cottage cheese with S&P, bananas, apples, vegies with dip, celery with peanut butter are some of the good choices he makes. Of course, if G'pa has Oreos or chips laying out, he hits on those. Sometimes I do too.