The finished project
Shaving cream "paint" still wet
I ended up just mixing the "frosting" paint in a ziploc bag, about 2 bottles of Elmer's glue to half a can of shaving cream. I cut the corner of the baggie off and squeezed the frosting on the cupcakes myself. The Kindergarteners were not able to use the small squeeze bottles efficiently, and ended up with not enough frosting, or way too much. Thus, I let them add more details, like the crinkles on the cupcake wrappers and the sprinkles and cherry.
The frosting paint took TWO FULL DAYS to dry. Beware of taking them off the drying rack and storing them when the frosting is dry-ish, even dry to the touch, it can still squish out from underneath and get them hopelessly stuck together.
UPDATE: I've since made this project even better by mixing in one more element: JELLO. See my blog on Chocolate Paint. After I tried it that way, I noticed the paint drying very hard, but it still had texture. I wanted the texture to be fluffy though! So I mixed about a half a box of JELLO pudding powder into a half a gallon ziploc baggie with a fourth a can of shaving cream and a half a bottle of glue. I made two batches; one chocolate and one vanilla. Below are the results of this experiment:
See how fluffy they are! They have to be staggered on the drying rack, because they even rise a little bit as they dry. Some students decided to make ice cream cones instead of cupcakes. I like that idea, too.
The finished product! If you look closely you can see a little indentation on the left side where the drying rack smushed the icing a bit as it dried.
So far I like the Jello/Shaving cream/glue paint the best. But for different applications, I can see how the other types could be very useful too!