Hello - I have found success by using giant plastic needles (used for yarn) and plastic canvas. I cut the grids into 4 inch squares and clean the edges so that they don't poke. They can "sew" any pattern that they want! I have several needles pre-threaded so that they can have as many colors as they would like. It's almost as easy as working with shoelaces. It really is great therapy for fine motor skills, color identification and its something that they can put down when they are tired and pick back up later.
Just an idea....