While they freezer paper is on the fabric, you can draw out the outlines, then use some heavy weight black thread, shorten the stitch length and sew around the black outlines. This helps keep the kids within the lines, and the black thread also will help keep the paint/ink from bleeding over if they hold it too long close to the edges :wink: the heavy black cotton thread acts like a wick, soaking up the extra.
Shortening the stitch length helps to remove the freezer paper once you are done with the blocks.