Mix together the baking soda, citric acid, Epsom salt, cornstarch, and butterfly sprinkles.
Melt the coconut oil in the microwave. Stir in fragrance oil and water. (The amount of water needed will depend on the humidity of your house.)
Pour the coconut oil mixture over the dry ingredients and stir with a whisk. The mixture should start clumping on the whisk.
Scoop up some of the mixture and squeeze it in your hand to see if a ball will form. If it does, the bath bomb mixture is ready. If not, spritz the mixture lightly with water, whisk, and check again. Continue spritzing and mixing until the mixture is slightly damp and will clump together. Again, this will vary depending on your climate.
Divide the mixture into four bowls. Color the mixture in three of the bowls with a tiny dab of food coloring.
Sprinkle some edible glitter stars in each of the two halves of the bath bomb mold.
Spoon some of each color of the bath bomb mixture into each half of the mold and pack it in with your fingers. Add more of the bath bomb mixture, overfill it, and lightly pack with your fingers. Then, tightly press the two halves together.
Carefully remove the mold from the bath bomb. Gently tap the mold if needed.
Repeat until all of the mixture has been used. I was able to make three small bath bombs.
Allow the bath bombs to dry uncovered. This will take a minimum of 24 hours.