Mix the citric acid and baking soda, breaking up any lumps.
Mix in the SLS or clay, then the fragrance oil and colorant.
If it doesn't hold together like damp sand, spritz it with witch hazel and mix until it does. Pack it firmly into lined cupcake molds and let them dry.
For the frosting, stir the meringue powder into the warm water, then add the cream of tartar and powdered sugar.
Whip on medium speed 4 to 7 minutes, until it holds stiff peaks, and pipe it onto the dry bath bombs. Let the frosting harden overnight.
Notes
Drop one into a warm bath. Store them dry and airtight. Safety: wear a dust mask when mixing SLS and citric acid. They look like real cupcakes, so label them and keep them away from small children.