This has to be one of the easiest (and messiest!) ways for children to produce a unique art piece.
All you need is paper (160gsm is good), poster paints and shaving cream (the cheapest brand works fine).
Make a layer of shaving cream on a tray. Make dots or lines of paint. Swirl with a chopstick or similar. The more you swirl, the more intricate patterns you will get, however if you overdo it you might muddy your colours.
When you have a pattern you like, carefully place your paper on top and lightly press all over (lightly!) Peel the paper off and use a paint scraper of similar to scrape the excess shaving cream and paint off to re-use. Put you paper somewhere to dry.
Keep re-using the shaving cream, reapplying the paint if desired, until it's too coloured to use. Then have some messy fun with the leftovers! (Good for making hand or foot prints on paper too).
This is messy. The easiest wash up for hands is to stick them in a bucket of water outside (the hands, not the children). The trays and tools wash up in warm water.
Here are some of our masterpieces.
They can be framed and hung.
Or used as wrapping paper or even for the basis of another project (collage, background for a picture etc).