I would try to include a little protein with whatever you give them, personally. Fruit and peanut butter or cheese. Pretzels (preferably whole-wheat) with cheese. Crackers with cheese or maybe a little sliced turkey. Soy crisps, if they'll eat 'em (they probably will, especially if you give 'em something to dip them in). Even some rice cakes or something with a little egg salad or peanut butter or a slice or two of turkey.
You basically don't want to give them just carbs; it'll hype 'em up for a bit and then they'll be tired and hungry - and cranky - later on. Same basic principles apply as with grown-ups, I think.
My younger cousins used to love those Easy Mac things - mac 'n cheese that you mix up and cook in the microwave. Probably not ideal, but it gave 'em a little protein and vitamins (and calcium, I think). Or they LOVED those little Laughing Cow-type cheeses. A couple of those with a few healthy crackers, and they were good to go.