Grease and flour two 8-inch (20-cm) layer cake pans.
Preheat ovento350°F (175°C).
To make cake, prepare chocolate cake mix with eggs, water and oil, if required, according to package instructions.
Spread batter evenly in cake pans.
Bake 35 to 40 minutes, following package instructions to test doneness.
Cool as indicated on package.
To make frosting, cream butter and powdered sugar in a medium bowl until light and fluffy.
Dissolve cocoa powder in water; stir into butter mixture.
Fill cake, using about a third of the frosting.
Cover cake with remaining frosting, swirling it with a spatula.
To decorate cake, make figures from marshmallows as illustrated; join with wooden picks.
Cut a few marshmallows in half and press 1 side into colored cake decors.
Place marshmallows and M&M's on cake.

Source:
http://www.ifood.tv/recipe/children-s-birthday-cake