If you can find the time, maybe make some mini veggie muffins. (You can freeze them too.) Grate some zucchini and/or apples and/or carrots and put them into a muffin recipe along with some flax seed meal. If your kids aren't allergic to nuts, you can throw some nuts and maybe raisins or oats into the mix too. Add half the sugar most recipes call for and use half whole wheat flour and half regular flour. You can also use olive oil instead of vegetable oil to make it a tad more healthy.
I have yet to try making them, but when I was in New Zealand, I once at the most delicious veggie muffin ever. From what I can remember, it had chunks of zucchini, onions, peppers, cheddar cheese, and a dollop of cream cheese in the middle. Maybe you could try something like that too.
Have you tried the freeze dried apple slices? Costco carries them. They don't have much in the way of fiber, but my kids love them. Those Go-Go Squeeze apple sauces that someone else mentioned are good too. A little pricey though.
You could also roll up some deli meat and cheese into a tortilla and have them snack on those.
Good luck! I hope more companies start making healthy snack choices for people on the go. I would certainly buy them!