1Heat oven to 350°F. Grease bottom only of 13x9-inch cake pan with shortening or cooking spray. Make and bake cake as directed on box for 13x9-inch pan. Cool 10 minutes. Run knife around sides of pan to loosen; remove from pan to cooling rack. Cool completely, about 1 hour.
2Frost top one-third of cake with yellow cupcake icing to make bedding sheet. Place flattened marshmallows on sheet for pillows; place cookies on top of pillows for heads.
3Frost remaining cake and over sandwich cookies with green cupcake icing to make blanket, leaving tops of cookies unfrosted for faces. Pipe hair and faces on cookies, using decorating icing.
4Outline blanket with frosting, using star tip; sprinkle with stars. Store loosely covered at room temperature.