I was recently given a child t-shirt from a realtor. It has the basic advertising logo on the front pocket but on the back is a black line drawing of a house with directions for the child to color the house with markers and how to make the ink set so it doesn't some off in the wash.
Nicholas had a lot of fun coloring his own shirt. I wonder if this would work with those iron-on transfer sheets that let you print an image from your computer and transfer it to a t-shirt? If so, the child could pick out some clip art that they just love from the oodles of resources online and then color the graphic themselves for a completely custom-made t-shirt. I just might have to try that!