Rather than having them sign their stars, could you have them sign the background fabric by the stars later? They could complete their star, put a piece of painter's tape or masking tape on it and sign it for temporary identifacation. When you have the background cut out and sewn (in a light colour fabric) tack down their stars and get a Pigma fabric fine pen and have them write their names on the background beside their star (then remove the tape)? It looks like there will be a small hole at the center of the star? I would attach the stars with a nice button sewn on through that spot.