When a small child approaches a PC for the first time and you want to give them a chance to play, what's the first program that you usually launch? I would guess Paint, which is exactly what I did for my three year old this week.
He loved moving the mouse and seeing the colors zipping across the screen, but I thought that there must be a better program out there for him to 'play' with and I found Art Rage 2.5
It's a graphic program where the tools try to mimic real art tools. The Paint Brush tool is not a single color but looks more like actual paint that 'globs' on and spreads thin as you use it. You can mix colors with the Knife tool and so on. You can add layers, stencils, and much more. It works with Mac or PC and there's a free download with some of the tools missing and it cost $25 for the full versions.
And most importantly, my son loves it ;)