I love these ideas! Especially #1 & #6!
To make this as safe as possible for your kids I would choose a safer-for-you soap (such as Ecover) for the ideas involving suds. I would also recommend a safer food coloring like the ones made by Color Garden or shop on Amazon from a company called India Tree Natural Decorating Colors. Finally I recommend finding a safer version of the typical plastic (PVC) shower curtain for #9. PVC, or polyvinyl chloride should be avoided because “hot steamy conditions promote the release of phthalates”. Here is an article to help you decide what to buy instead.