Special equipment: a 4 1/2-inch-round cookie cutter; 2 piping bags fitted with a round number 3 tip; 2 small piping bags
Preheat the oven to 350 degrees F.
Prepare the cookie dough according to the package directions for roll-out cookies, incorporating flour as instructed. Cut the dough into twelve 4 1/2-inch circles, rerolling the scraps as needed. Arrange on 2 ungreased baking sheets. Bake until the edges are light golden brown, 11 to 14 minutes. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely, about 30 minutes.
Meanwhile, combine the confectioners' sugar and meringue powder in a large bowl. Add 5 tablespoons water and beat with an electric mixer on medium-high speed until the icing forms thick and glossy peaks, adding up to 1 tablespoon more water if needed, about 2 minutes.
Transfer 1/4 cup of the icing to a piping bag fitted with a round number 3 tip. Spoon another 1/4 cup of the icing into a small bowl and thin with 1/2 teaspoon water to flooding consistency. You will know it has reached flooding consistency when you run a knife or fork through the icing and it flows back to where it was in 10 seconds. Add more water 1/4 teaspoon at a time until you've reached the right consistency. Transfer to a small piping bag (no tip needed).
Add the food coloring to the remaining icing in the bowl and mix to combine. Transfer 1/4 cup to a piping bag fitted with a round number 3 tip. Thin the remaining blue icing with 1/4 teaspoon water to flooding consistency. You will know it has reached flooding consistency when you run a knife or fork through the icing and it flows back to where it was in 10 seconds. Transfer to a small piping bag (no tip needed).
Using the thick blue icing, pipe the outline of the ocean and some waves on the bottom half of each cookie circle and let dry, about 20 minutes. Fill in the ocean with blue flooding icing using a toothpick to spread it all the way to the edges. Let dry completely, about 30 minutes.
Outline the top half of each cookie with the thick white icing. Let dry completely, about 20 minutes. Fill in the empty space on the top half of each cookie with the white flooding icing using a toothpick to spread it all the way to the edges. Sprinkle 1 tablespoon crushed graham crackers over the wet icing on each. Let the icing dry completely, about 30 minutes, then tap the cookies to remove any excess crumbs. Pipe white ocean foam in swirls over the space where the blue and sand meet. Let dry completely, about 20 minutes.
Meanwhile, build the sandcastles: Dip 4 mini marshmallows in water, then roll in the graham cracker crumbs until completely covered. Use the thick white icing to stick 3 marshmallows together to form the base of a sandcastle. Pipe a small amount of icing on the bottom of the fourth marshmallow and stick it on top of the base. Repeat with the remaining marshmallows, cracker crumbs and icing to make 12 castles.
Make the flags: Use a sharp knife to cut 12 small flags out of the rainbow sour candy. Cut 6 toothpicks in half for the flag poles. Stick each flag to a toothpick piece, then stick a flagpole into the top of each sandcastle.
Make the sand buckets: Cut 12 strips from the green taffy, each 1/2 inch long and 1/16 inch wide. Roll 1 strip in your hand to a 1-inch-long rope for a bucket handle. Use thick white icing to glue the handle to a green gumdrop half. Sprinkle some graham cracker crumbs into the top of the gumdrop bucket. Repeat with the remaining taffy strips, icing, gumdrop halves and graham cracker crumbs to make 12 buckets.
Make the shovels: Cut 12 strips from the green taffy for the shovel handles, each 1/2 inch long and 1/16 inch wide. Cut twelve 1/4-inch squares from the green taffy. Squeeze one square onto the top of each handle to make 12 shovels.
Make the crabs: Use the round part of a number 3 piping tip to cut 12 half-circles out of the red taffy. Cut 72 strips for legs, each 1/6 inch wide and 1/4 inch long. Press 3 legs onto each side of each half-circle to make 12 crabs.
Make the seashells: Use a sharp paring knife to cut 2 small pieces off the bottom of each white chocolate chip to create a seashell. Use the paring knife to make indentation lines in the shape of a seashell on top of each chip.
Assemble the cookies: Use thick white icing to glue 1 sandcastle, 1 bucket, 1 shovel, 3 seashells and 1 crab to each cookie. Let dry completely, about 20 minutes.
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