I've seen many "recipes" on the web for this so I made one I vaguely recalled using clear bubble bath and cornflour, along with some food colouring. I kept the food colouring to a minimum, just in case it stained anything but it was fine - in fact, the colours were a little muted so I would probably used some more next time.
Add two paintbrushes and some eager children then let loose on the bath tiles!
They loved it, drawing pictures and writing letters on the walls. Plus they had just as much fun washing it off the walls with the shower afterwards!