2 Tbsp. cornstarch
1/4 cup liquid dish soap
4-5 drops food coloring
3 Tbsp. water
In a muffin tin, add cornstarch (ingredient amounts listed are per muffin tin cup). Mix dish soap and food coloring; stir until smooth and color is fully incorporated. Add water and stir.
Give each child a paintbrush and let them paint the bathtub while bathing or just before bath time.