Great Examples

The Gray Tree by Piet Mondrian

"Negative space need not be boring.  This tree is a great example of that.  Mondrian attracts our attention  to the negative areas with this brushwork, an makes these shapes as much the subject of the painting as the form of the tree."
-Design Basics

By Piet Mondrian

This piece by Piet Mondrian is a fun example of shape.  If you think your students would like to do something like this the book How to Teach Art to Children has instructions on how to do so on pages 112 & 113.

     The Violin by Juan Gris

This piece of the Violin is excellent to show what shape can do in art.  Notice the amount of rectangles.  Then just by using the one object with rounded sides (the violin) he makes that his focal point. It is the first thing your eye looks at. How simple.

These three drawings by Gorges Seurat are demonstrating three types of positive/negative shapes with shading.  If you are interested in teaching a little more on this type of an idea, I would be happy to help you out, just let me know.