One of my friends was recently telling me about how she makes her own bathtub paints. I thought it sounded like something my kids would love so I looked online to see if I could find a recipe. I found this one on FamilyFun's website. I used paste food coloring and ended up having to scrub the grout with soft scrub w/bleach. I think you would be OK just using a little bit of regular food coloring. However you may just want to have your kids paint the tub and themselves to avoid staining your grout! :) My kids LOVED it and can't wait to do it again.
Here is the EASY recipe!
1/2 cup liquid hand soap, clear or white
1 tsp. cornstarch
Combine the soap and cornstarch in a small bowl. Divide the mixture into separate containers, then add food coloring one drop at a time. Apply to tub or body with fingers or paintbrushes.