More to explore:
homemade bath crayons. other toddler crafts here too--melt glycerin soap plus food coloring, cooled in ice cube trays
Easy Homemade Bath Crayons- Buy some glycerin soap (find it at a craft store by my grocery store has it also). Melt it in the microwave, add food coloring, and pour into a mold (the picture shows ones from an ice cube tray). Could also add essential oils if you go for scents, though it might be a bit unnecessary for kids bath crayons. Takes about an hour to cool completely, pop out of the mold and there you have crayons!