I think something like this would actually make for a really interesting 3rd grade class project, with each student contributing a few pixels.
When completed, it would show a QR code, that upon scanning would mark completion of the workbook and show you the answer key. The implicit idea being you didn't have to be perfect, just good enough so the error-correction algorithm of QR codes was enough to pick up the final image. (Probably extra credit if you could figure out how to do as few problems as needed and then use EC to still figure out the secret link)