I also thought this would make a really neat Birthday card. Do a coloring picture of the birthday boy/girl on the front of the card and give a few crayons inside.
If you don't have the tools I use for the computer you can also just lay a blank sheet of paper over a blown up photograph and trace with a thin black marker.
Update 10/2/12 : I do know there is an effect in photoshop that does something similar but decided that there was too much cleanup involved since I didnt want the background, only the basic outline of the kids. If you do not have a wacom tablet and want to do this use the effect instead :)