I have seen this on a couple of blog such as One Crayola Short, and Art Projects for kids. If you aren't already, you should become a follower of these two great blog! I thought this project was perfect for this my 4th grade. I'm always amazed at how excited they are about Op Art. One resource new to me this year was Optical Illusion Card by Usborne books. I was able to use them at tables and on my doc cam.
Check out the demo of my sample project. Just hit play!