To answer my own question, I found a great lesson in a book I had titled
"Art for the Very Young".
They suggested reading a book to them "Round Trip" by Ann Jonas, and WOW
did they love it!! Neat black and white illustrations that are one image
right side up and another image upside down. Then, they each get a white
paper with some black shapes to glue on in a design of their choice, and we
talk about where the "shapes" are, and where the "space" is. Next they
each get a black paper with white shapes and repeat. They loved it!