I have two small children and when my son was 3 (a year ago) he saw me using sketchup and asked if he could play that "drawing game" so i just showed him a couple of the tools like the eraser and pencil tool and then how to drop in components. He picked it up very quickly, although most of his "work" seems to be abstract. He does have fun for 30 to 40 min and then is off to other things. Also, at 3 he began playing these nickeloden puzzle games, such as dora, spongebob, jimmy neutron, etc. after putting a couple of these together i noticed that he would open a puzzle and while it was still unworked he would change the background in the puzzle game to the same color as the predominant color of the puzzle... and then he would minimize the game and change the computer's desktop background color to the same color. I was floored, and asked my wife who taught him to do that. She said noone, just something he picked up. It makes me wonder sometimes about people complaining about intuitiveness of the OS's maybe it's our preconceived notion of what it should be that clouds our ability to pick something up.
My daughter who is 18months younger really doesn't gravitate to the computer like my son, she just wants to paint stars with my wacom when she sees photoshop open.... to each their own.
after thought also, i'm not ashamed to say that my son showed me that the eraser tool can be used by clicking and holding down the mouse button and dragging the cursor around to delete what you want... i was just picking individual entities and deleting one at a time.
so, now i get made fun of that i was actually taught about software by a 3 year old.