In advance, using stiff royal icing, figure pipe children. Hold bag at a 90° angle and pipe tip 2A in circular motion to form inner tube if needed. Pipe tip 2A ball torso and head. Tuck tip 9 into sides, then pull-out arms and legs; add tip 2 dot hands, feet, fingers and toes. Pipe in tip 4 swim trunks, (smooth with finger dipped in cornstarch). Pipe tip 2 dot flowers with tip 1 dot centers. Pipe in tip 4 bathing suit top on girl (smooth with finger dipped in cornstarch); add tip 2 ruffles and bow around neck. Add tip 3 dot noses and pipe in ears. Add tip 1 outline or dot eyes and tip 2 outline mouths. Pipe tip 2 curly or pull-out hair.
Ice smooth 2 oval cakes, each 1 1/2 in. high. Ice sides smooth in dark blue, top in light blue. Pipe tip 48 ribbed side up bands 1 1/4 in. apart, for support beams on cake sides. Pipe tip 2 swirls on cake sides. Edge top border with tip 2B band; let icing crust, then overpipe tip 48 ribbed side up support bands.
Add blue piping gel water on cake top. Sprinkle brown sugar around cake. Position piped figures and candles.