Combine baking soda, salt/epsom salt and mix well until combined.
Spray or drop small amounts water into the mix and stir until you get the consistency of slightly wet sand.
Mixture should hold together when squeezed without crumbling but should not appear wet at all. You may need to add slightly more water if it hasn’t achieved this consistency yet.
Once you get this texture, add the menthol crystals and the essential oils and stir to combine.
Quickly push mixture into silicon molds, greased muffin tins or any other greased container. Press in firmly and leave at least 24 hours (48 is better) or until hardened.
When dry, remove and store in air tight container.
Notes: Please use caution with products like menthol and strong essential oils like peppermint or eucalyptus on small children. As a general rule, I personally use these only on children 2+ and I dilute them but check with a qualified health practitioner or aromatherapist before using anything on small children!