This is a great way to paint with toddlers, just put a few drops of food colouring in some water and go for it. It's washable, edible and creates a really lovely silk screen effect. If you don't have paintbrushes you can just use ear buds instead.
At this age they really don't stick to one activity for long so it can be frustration putting in a whole lot of effort only to have it snubbed two minutes later. This is quick and easy, I just keep the little cups of 'paint' on the windowsill so it basically instant.
Best enjoyed nude, and outside if possible.