How about painting onto food? :-)
Yes, that is a real slice of bread!!
I'm a primary (elementary?) school teacher and I did this project in mind to do it with some of the classes that I teach in. I have successfully done it with a Year 5 (9 year old) class ... the finished art looked amazing! Most of the kids painted eggs, one did jam on toast; some did different little crazy things, like taking a bite out of the toast, making the egg spill over the toast onto the plate, double yoke egg, etc.
The toast is varnished so it will never go moldy. I silver leafed the knife and fork.
I didn't work too long on making the egg look realistic, but that is the idea. Be keen to see what else people find to paint on besides paper or canvas.