David Laferriere is a graphic designer and illustrator. He's also an awesome dad who has been drawing stuff on his kids' sandwich bags since May 2008, using Sharpie markers. In his own words:
Each drawing is done just after I make the sandwich. I take a picture and post to flickr. My kids don't see the drawing until it is lunchtime. The challenges are coming up with an idea and then drawing quickly and directly on the bag, every line counts."
Indeed it does. Your kids must feel the same: every drawing counts.