Let the kids tear apart the crayons until they are in tiny pieces. Parental supervision is recommended. Insert a piece of clean white paper between the layers of the T-shirt.
Kids can create a pattern on the T-shirt with the broken crayon pieces.
Simple patterns might include a heart, a peace sign, or a rainbow. The adult can place waxed paper on top of the crayon pattern, and a dish towel on top of that. Use the iron on a low to medium heat to melt the crayons into place. Since the crayons are colored, the idea is to melt the color into the fabric.