Reserve 1/2 cup of the frosting. Tint the remaining frosting green, frost the cake, then sprinkle on the green and white sugars.
To make the mushroom, trim the top of the cupcake so that it's flat, then place it upside down on the cake. Top the chocolate cookie with white frosting, red sugar, and white candies, then attach it to the cupcake with a dab of frosting.
For the butterfly wings, frost the back of the pretzels so that the frosting pokes through the holes. Sprinkle on colored sugar or nonpareils as shown, place the wings on the cake, and add the oblong candies.
Make flowers by pressing sections of rolled wafer cookies into the cake, inserting a toothpick in the center of each section, then threading on a peach gummy ring and Dot candy. Press some flowers without stems directly onto the cake. (Note: Be sure the toothpicks are removed before children eat the stemmed flowers.)
Decorate the rest of the cake as shown.