Here's a fun method that turns your child's artwork into a wearable masterpiece. Just remind him ahead of time that the process requires placing drawings facedown on the fabric, so letters need to be printed backward to transfer correctly.
Have your child sketch a design on white paper and then color over it with the fabric crayons. Advise him to apply pressure when coloring (brighter colors transfer best) and to shake off excess crayon specks when he's finished. If he wishes to include letters, he can first print them on paper with a black permanent marker, then turn the paper over and color in the reversed outlines with the crayons.
To transfer the design, stack newsprint on an ironing board, lay the shirt on top, then layer paper inside the shirt to keep transferred color from seeping through. Place your child's drawing facedown on the shirt, then top it with a sheet of paper.
With a dry iron on the cotton setting, press straight down on the design with a firm motion until the image is slightly visible through the paper. Peel away the drawing and launder the shirt according to the crayon manufacturer's directions.