You'll need a bottle of glue for each color you want to use. Add a few drops of food coloring to each bottle to create custom colors, then close and shake the bottles vigorously to make sure the coloring dyes the glue evenly. Add the glitter, if you're using it, and shake again to distribute it evenly.
Spread out your wax paper, and you're ready to create! Kid can either draw their designs freehand or use the cookie cutters as molds for their window clings. You can even sprinkle a bit more glitter on top of your art before it dries, if you like.
Let your creations dry overnight, then peel them off and stick them onto the bathroom mirror or windows.