In advance: Using royal icing, figure pipe kids with tips 2A, 3, 5, 8, 12, 16 and 349. Make trees using royal icing: Place cones on waxed paper. Cover cones with tip 16 pull-out star branches, piping from bottom up. Using royal icing, make campfire: Position mini pretzels in a circular stack on waxed paper. Pipe tip 349 pull-out flames. Let all dry.
Trim off keys on neck area of guitar cake. Place cake on foil-wrapped board extended 4 in. past neck area for piping "water." Ice cake smooth in buttercream. Pipe boards and corner poles on dock with tip 45 bands. Pipe tip 8 ball rocks and tip 233 pull-out grass. For tents, use knife to split wafer cookie into 3 flat sections. Remove icing and cut sections into 2 1/2 x 2 in. pieces. Attach at an angle with royal icing; let dry. Tint piping gel blue and spread on board for water. For tallest tree, insert pretzel rod into cake. Pipe tip 16 rosette on top and position tree. Position kids, tents, trees and campfire.