My son likes trail mix made of granola cereal with almonds and raisins. You can add mini M&Ms in small amounts, or you can add carob chips from the health food store (tastes like chocolate). He also likes a granola cereal made with dried raspberries. He likes carrots dipped in peanut butter - I put peanut butter in small plastic containers and he can just grab one along with the mini carrots. Keep celery cut up in a container of water in the fridge. That's good with peanut butter or a low fat spreadable cheese. Some kids like cut veggies dipped in ranch dressing too. It satisfies the need to "crunch" without them always eating cookies or crackers. BJ's sells bags of broccoli florets already cut up - some kids will eat those raw. Can you use hummus as a dip? Theh protein is great. You can also make a sort of Chex mix without all the fat and salt by using whole grain Chex, whole grain cheerios, mini pretzel sticks, peanuts, and a healthy butter substitute like Smart Balance. Follow the Chex Mix recipe on the back of most boxes of Wheat Chex, but make substitutions for healthier choices.
My son also likes the South Beach Living High Protein Cereal Bars - they aren't cheap but they are great for packing in a lunch or for taking in the car. At least they are heavy in the protein and light on the sugar. If you can keep the kids' protein level up, they don't get the spikes from too much sugar, then the crash, and then more hunger pangs!
The Laughing Cow Light cheese wedges are pretty low calorie. They are good alone, or spread into a celery stalk. Celery actually takes more calories to chew and digest than it contains, and the cheese provides protein.
If you can boost the protein in their meals, they may not get as hungry as quickly. Same goes for using complex carbohydrates (whole grains, for example) instead of white flour. I agree with the other posts about not using high fructose corn syrup.
Some recipes my families loves are using whole grain couscous or brown rice, even mixing plain brown rice in with a box of your favorite boxed rice pilaf or other mix. Double the water, and use a plain rice - it cuts the salt of the mix by half, but still gives you the flavoring/spices. I even save the water from steamed vegetables - all those green goodies and vitamins! I freeze it, then defrost it as needed and use it for any recipe in which the water is completely absorbed, such as rice. Don't waste it on pasta where the majority of the water gets drained off. My son likes boxed falafel mix - I stretch it and cut the salt by adding wheat germ, flax seed, sesame seed, etc., and adding more of the veggie water. I even put spinach in it and tell him it's parsley! I make patties rather than round balls because they cook faster. I fry them in olive oil on both sides until crispy, then finish in the oven so they are baked. My son also likes "home fries" done in the oven - cut potatoes into spears, drizzle with olive oil, season as desired (pepper, a little salt, dill, parsley, whatever) and bake until fork-tender. You can quarter an onion for extra flavoring, and even mix half white potatoes with half sweet potatoes. Again, these are all things to fill the kids up and hold them to reduce the need for snacks.
Have fun!