While I have no qualms about eating a sponge cake as it is without any frosting, the kids are more than eager to decorate a cake first before eating it.
It is a really easy activity, with only 4 items:
/ a 6″ or 8″ cake of your choice (plain cupcakes or muffins from confectioneries are also good alternatives and just as fun)
/ whipping cream (whipped cream frosting instructions)
/ food coloring: frost the entire cake with plain whipped cream first, then color the rest in separate bowls.
/ some plastic bags diy into piping bags : fill the bags with frosting cream, squeeze it down to one corner, twist to tie up the bag, making a knot closer to where the cream is and cut off a tip as a piping hole. Try to cut a smaller hole at first. You can make a small hole bigger if you find it too small for the cream to be piped out smoothly, but you cannot turn a bigger hole smaller.
The rest is up to their imagination and creativity. Have fun!