How old are the kids? When I taught preschoolers, the BIG hit were donut holes. Parents would ALWAYS bring several dozen donut holes, and those things were GONE. Brownies, cupcakes, and cookies weren't touched. Krispy Kremes are another good idea.
Brownies
Rice Krispy Treats (I love the idea of frosting them!)
Brownie Bites
Twinkies
Cookie Cake you can write a message on (EASY to make with prebought dough)
Cookies
=)