Make your own "trail mix" of what they love. Or you can have a few choices and they can help make up baggies daily or weekly.
goldfish
pretzels
nuts (if allowed)
popcorn
m&m's
choc chips
chex
cheerios
raisins (even yogurt covered raisins)
dried fruits (like cranberries, cherries, blueberries, or tropical blend)
rice crackers
sesame stix
etc....
Or a small amount of granola to top yogurt cup
ranch cups with veggie stix
PB (if allowed) with graham crackers, carrots, or apple slices
grapes or other fruit
Lots of choices, but fun would all depend on your children's likes and dislikes.