Split the cookies open and scrape off the appropriate amount of cream filling, so you're left with the desired "picture" of the moon.
You could use a whole bunch of these and create the whole cycle. Use some frosting to "glue" the cookies to a plate in the appropriate order.
Or, you could use it as a quick, fun assessment - give each student one cookie and have them pick one of the moon phases out of a hat. They then need to use the cookie to illustrate the phase they selected.
