I always back fabric with freezer paper (8" square seems about right for the little ones) and have them draw their creations right on the fabric with Pentel Dye Sticks. Cheap and fun to use.... www.dickblick.com has them for a couple of dollars a box. Iron the blocks to set the dye, (use a paper towel between block and iron) and you're set to go. I did one of these quilts about 20 years ago and though it has frayed places on the binding and around in various places, the colors on the blocks are still bright!