If you have watermelon juice already made, add it and the lime juice into a jar with a spout. If you don't, add the watermelon cubes and lime juice into a blender and blend, then strain. Here's a guide to making watermelon juice.
Pour the juice into popsicle molds, leaving 1/2 inch at the top for expansion when the juice freezes.
Add in a gummy candy (optional) to each mold.
Place a popsicle stick inside the mold and attach the lid.
Freeze overnight. To remove in the morning, run warm water over the bottom of the mold to release the popsicles.