My children Manuel (6 at that time) and Leo (3 then) did a piece of silk painting.
I first outlined a lotus flower with silver gutta. Then I pinned this hankie scarf (9 x 9inch or 25 x 25cm) to a paper carton. Voila, boys played around with colours.
It took 10 minutes for them to finish the painting. However, preparing and cleaning their hands took me the heck of time.