Tux Paint (http://www.tuxpaint.org/) is a free, award-winning drawing program for children ages 3 to 12 (for example, preschool and K-6). It combines an easy-to-use interface, fun sound effects, and an encouraging cartoon mascot who guides
parents children as they use the program.
Kids are presented with a blank canvas and a variety of drawing tools to help them be creative. One of our favourites is the stamp tool where the child can take an existing picture (for example train, animal, tree) and stamp them on the canvas. Similar to what you would do with a rubber stamp (not the postage stamp...). You can download various stamp bundles online. As a train enthusiast, Josiah's favourite is of course the train set.
The freehand drawing tools are also good fun, definitely much easier to use than those in GIMP or Photoshop.
I recommend that you give it a try, let your (inner or actual) child unleash his/her creativity. And did I tell you that it is free?