Little Painter is a simple paint app for kids. It has a bright interface, and even makes sounds as you paint (this can be turned off).
Little Painter is very similar to the classic Paint - so don't expect any Photoshop-like features. It's a simple app which lets you sketch, with a couple of good features kids will enjoy - for example, you can convert your drawing into an outline which you can print for coloring in later. You can also download some prepared outlines to color in.
If you want a simple desktop paint app for kids, Little Painter will do. It's a bit rough around the edges, and seems to come from the Windows XP era, but it works.