I like lead by example and simply provide the right stuff and don't buy the wrong stuff. Yes, they'll eat cr-p when they go to their friends' houses, but just make it there for them at home. My father's favorite snacks were apples, which he carefully cored, melons (not like now, when you can get them any time). Big junk food binge? A can of fruit cocktail and scoop of ice cream. But only as an occaisional treat!
My nephews became healthy eaters when they started to like dipping things. So a sliced apple and a bit of some king of dip (fruit or honey yogurt comes to mind) just made them happy.
A lot of children like child-sized portions. That is to say, fruit (fresh or dried), nuts, vegetables, etc, put in small bags so that whatever they have is theirs and theirs alone. Buy some of the smallest, cheapest baggies, and make them a snack that just belongs to them (if you have a lot of kiddie buddies, make them a bag as well, to keep on hand).
But if Mom and/or Dad are sitting on the couch, watching TV, eating garbage, then don't expect the kids to be out playing in the sun and eating fruit.
Oh, another good one is a baked potato. Sweet or regular, with some toppings, for dinner. Just don't turn the sweet potato into a desert type dish. Just bake it.