Combine bath bomb base mixtures together. Whisk to combine.
Add epsom salts. Whisk.
Combine oil and soap/food coloring together in a small bowl. Add to dry ingredients and whisk.
You want your mixture to feel like wet sand. Spritz your mixture with water or rubbing alcohol, making sure to whisk in between spritzes, until it reaches the desired consistency. DO NOT add too much water. You just want it to come together slightly when you press your fingers into it.
Place soap mold on a baking sheet.
Add a small amount of bath bomb mixture into 4 soap molds. Press. Add a small eraser or toy.
Pack each soap mold with remaining bath bomb mixture.
Let dry in a DRY PLACE for 24-36 hours.
Store bath bombs in an air tight container.
Bath bombs, freshly made, are very delicate. They will thoroughly dry out after 2-3 days and can easily be stored in a container without breaking.