As a preschool teacher, my suggestion would be to limit treats to low-sugar/sugar free snacks. This isn't about being a meanie, but helping families avoid meltdowns and sugar crashes. If families aren't present, think of the teachers. A room full of tired, whiny sugar-crashing kids? Sooo un-fun! (Some will disagree with me on this, say that sugar has no negative affects, but I know what I've seen...)
Kids love: apple slices, popcorn (love love love it!!!), almonds/nuts (if there's no restriction on these for allergic reasons), strawberries, string cheese cut up into circles, grapes, crackers, baby carrots/veggies for dipping and hummus or ranch, bananas, raisins, etc.
I've noticed that when we have these sorts of kid-centered events, parents like to bring on the sweets. The trouble is, this is all the kids usually eat. They don't often sit down and have the more substantial foods that would help them better process the sweet stuff. Popcorn is my preschool's favorite: we have a "popcorn picnic" each Thurs at snacktime with popcorn and apple slices served on a blanket on the kitchen floor, and the children always ask me "when is popcorn day?" on a daily basis. So, if you are wanting success, if it were me, I'd start with popcorn.