Dirt Cake is such a fun project to experiment with children. It is suitable for celebration like Earth Day, Easter (with carrot cookie), Halloween (with scary-looking worm), and many others. Its recipe is relatively simple with no baking involved. It is a matter of time for chilling and decorating the cake!
Original Recipe:
http://www.browneyedbaker.com/2013/05/20/dirt-dessert-recipe/
http://womenlivingwell.org/2010/07/tasty-tuesday-dirt-cake/