I was about to buy some for my kids to play with in the bathtub when I was thunderstruck by the power of google. I found several recipes, then changed them to actually work and here it is.
Mix equal parts bubble bath and cornstarch. 1/4 cup of each is enough for one kid in one bath.
The great part about the bubble bath is that it comes pre-coloured so you don’t have to add food colouring. If it’s not dark enough or not the colour your kid likes, add food colouring.
I was apprehensive about adding food colouring in case it stained the bathtub, but it’s mixed in with so much soap that it doesn’t stick.
I gave each kid a cheap paintbrush in turn to paint with. It just wipes right off when you’re done.
Another dispenser option is an empty syringe. Ask for a few free ones at your local drugstore. They don’t have needles in them, they’re just the squirty bit.