H.W.
As a preschool teacher, I have to kindly request: Please Do NOT send Rice Krispies treats. There's nothing worse than a room of kids trying to cope with the sugar in that snack. Meltdown city!
That said, if it were me, I'd do sandwiches. Something simple like almond butter and jelly or cream cheese and jam... You can even cut off the crusts, so the bread is a square shape, and you can cut halves and quarters from each sandwich, so you have the option of either shape. The kids readily eat sandwiches, it's simple and doesn't cost a lot. And still sweet.
Have fun,
H.