Tux Paint is a painting program aimed at children from 3 to 12.
Despite the young target audience, Tux Paint isn't short of features. In fact it has more functionality than some regular paint tools: 50+ brushes, line and shape tools, customisable stamps, text and label tools, and 60+ special effects (add raindrops, snowflakes, rainbows, grass, confetti and more with a click).
What's different here is the interface. It's visually a mess, but that doesn't matter: there are sounds as you click things, text prompts explain what to do next, a cartoon character walks you through every step. Children won't necessarily pick up what to do immediately, but they're encouraged to experiment, there's immediate visual feedback for every step, and of course an Undo button to recover from any mistakes.
None of this seems to be even faintly configurable, but that's because the program's settings are kept safely away from its users. A separate Configure program enables you to set its window size, mouse and keyboard use, or print and save-related options. A Simplification tab provides several interesting interface tweaks, including disabling the Quit button (a good idea if you want to leave a young child in front of the Tux Paint and know they can't easily shut it down).
There are some other advanced tricks, if you need them, so be sure to check out the program's Readme file for all the details (\TuxPaint\docs\html\README.html).
A fun way to get young children using your PC.