Bake and cool cake. Place on foil-wrapped cake circle 1 in. wider than cake. Ice cake in blue buttercream icing. Divide cake into 12ths.
Use the smooth side of tip 47 and white buttercream icing to pipe vertical stripes at divisions on sides of pool.
Use the smooth side of tip 47 and white buttercream icing to pipe a stripe top and bottom border on cake.
Cover top of cake with blue Sparkle Gel. Pipe an additional tip 47 stripe around top edge of cake.
Insert heads in cake. For outstretched arms, insert spaghetti noodle where arms are positioned. Using tip 9 and copper buttercream icing, pipe over noodle. For hands, pipe tip 9 round balls; use a small knife dipped in cornstarch to score and form fingers.
For boy with arms leaning on pool, use tip 9 and copper buttercream to pipe arms. Use tip 9 and copper buttercream to pipe round balls at end of arms, use small knife dipped in cornstarch to score and form fingers.
Use tip 233 and green buttercream to pipe pull-out grass bottom border.