Prepare and bake cake according to package directions, using two greased 9-in. round baking pans. Cool for 10 minutes before removing from pans to wire racks to cool completely.
Spread frosting between layers and over top and sides of cake. Lightly press the Nerds, sandwich cookies and animal crackers onto sides of cake. Arrange marshmallows along edge of cake. Just before serving, arrange cotton candy and lollipops on top of cake.