Most pre-schools (and most schools) can only allow commercially packaged treats. It has to do with the health department regulations - they can only serve things that were prepared in a health dept certified kitchen (this includes treats parents bring/send). So take that into consideration.
Maybe a non-food treat would be better. Preschoolers love stickers. Or those little magic-sketch pads, or something else like that. Don't feel like you have to do a food-treat to celebrate her birthday (unless you're the official treat-bringer for that day and they're counting on a food; then you want to make it more healthy and less sweets.)