Kids 10-14 making a matte background? It's exactly what I'd recommend. It's clean, simple, and easy, and allows everything from basic painting to per-pixel modification. No, it's not vector like Inkscape, or able to do complex multilayer tasks like GIMP. But it uses the standard Windows interface, is simple to use and understand, and is capable of astonishing levels of detail for the really attention-fixated. If the kids were older, or the task more complex, it'd be far underpowered, but I've seen people do some unbelievable things with nothing but Paint.
That said, I'd never use it. For basic matte painting when you're not an image-processing expert, the best tool I've found is Photoshop 5. But it's a little hard to find.
I keep my own copy backed-up just because I know it'll someday be impossible to locate, and for beginners, it's a hell of a lot better than CS3.