There are plenty of alternatives to junk food that kids love:
Instead of potato chips, have a handful of whole-wheat pretzels or baked tortilla chips; baked chips have a fraction of the calories and much less fat than their deep-fried counterparts. Serve with hummus or yogurt-based (instead of sour cream) dips.
Instead of packaged trail mixes containing candy or high fat cereals -- it's simple to make your own trail mix tailored to their taste preferences. Combine a low-fat/low-sugar granola with a few nuts and dried berries or raisins. It's easy to make a batch and divide into servings in small plastic bags for a non-perishable and very portable snack.
Instead of the microwave popcorn packets that offer convenience along with hydrogenated/trans fats, lots of preservatives and too much salt -- use an air popper or pop the corn in a small amount of olive oil and season lightly.
Instead of sugary soda -- mix 3 oz of unsweetened fruit juice with 3 oz of sparkling water and a slice of orange.
Instead of milkshakes at the drive-through, make smoothies at home. Toss a banana or berries into the blender with milk or juice and some yogurt for a frosty breakfast or after-school treat. Keep straws on hand and call it a milkshake.
Instead of candy - keep frozen grapes in the freezer. They're sweet, crunchy, fun to eat and loaded with antioxidants.