This is a project I just did with my preschool class, but it could be adapted for older ages. Kids love the bright colors, funny shapes and lines Miro uses in his work. I like using Abstract art to teach kids that you don't have to be able to draw something realisticly to come up with a great piece of art.
8"x10" board (or you could use paper)
white, blue, green, red, black, yellow acrylic paint
For the preschool class I pre-painted the board white, and pre-drew the large shapes (see sample). If this were a kindergarden class or above I would let them do all the drawing. I had the preschoolers sand down the board. Then I had them draw in the other lines and shapes with a pencil, while I demonstrated it on a whiteboard. We talked about the different kinds of lines and shapes in this piece and how to make them.
They went over all the lines in Sharpie.(on my sample below I skipped the Sharpie part). Then we started with the lightest color (yellow) and filled in all the shapes-giving the yellow, red, green and blue areas two coats. After everything was dry we went back in with a small round brush with black paint and went over all the lines and filled in the circles.
The results were great and the kids had fun. (Unfortunately I didn't have my camera).