Try changing one element of the paper you offer to your children to create their artwork on. It is surprising how a simple change can alter their process and result.
In this case we have changed the color of the paper for the artwork, this makes even a simple picture a colorful artwork. I think that a display of these in all different colors on a school wall would look pretty fabulous!
Another similar example from a while ago that you may also want to try is Holey Paper for Children’s Art.