GIMP 2.6 seen in Microsoft Windows.
Now, just because the title says it's not just for kids, it doesn't mean they can't use the software. After all, they can't break it. And what's the worst that could happen? They learn something about themselves, express themselves, and maybe even surprise you in the process. With that said, here are a couple of art programs that even though they may be free or you may not have heard of them, they can hold their own.
GIMP is probably the best known of the open source art software. Its similar to the more commercially known image manipulating software and there are even add-ons that can make GIMP look pretty darn close to the user interface of the commercial version. GIMP comes with features like Customizable Interface, where you can make the screen real estate work to your advantage and comfort. Photo Enhancement, such as fixing Barrel Distortion, Perspective Transform, and Sample Colorize. GIMP is extremely effective in advanced photo retouching techniques. GIMP includes a very unique support for various input devices out of the box, from pressure and tilt sensitive tablets to a wide range of USB and Midi controllers. GIMP even supports a wide range of file formats. From the common JPEG, GIF, PNG, and TIFF files to special use formats like the multi-resolution and multi-color-depth Windows icon files. Also GIMP imports files from Photoshop (psd) and can also read scalable vector graphics (svg) files. You can even use plug-ins to add support for rare file formats. GIMP is supported on the GNU/Linux, Microsoft Windows, Mac OSX, Sun OpenSolaris, and FreeBSD operating systems.
Scribus seen in Windows7
Inkscape is an open source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format. Inkscape supports many advanced SVG features like markers, clones, alpha blending, etc., and great care has been taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps, and much more. On Inkscape's screenshot page, there are examples of fantastic work created in Inkscape. This program is very versatile and has all the bells and whistles that the commercial programs have, plus some extras. It's worth your time to check Inkscape out.
Scribus is an open source program that brings award winning professional page layout to Linux/UNIX, Mac OS X, OS/2 Warp 4/eComStation, and Windows desktops with a combination of press-ready output and new approaches to page layout. Underneath the modern and user friendly interface, Scribus supports professional publishing features, such as CMYK color, separations, Spot Colors, ICC color management, and versatile PDF creation.
If you are interested in 2D and 3D software then you may want to check out Synfig and Blender, respectfully. Both programs are open source, supported on Windows, Mac, and Linux, and can produce some amazing works of art.
Donating to the community that made your open source software choice helps greatly as well. It keeps the creators working and updating the very software you downloaded and are now depending on. Help them help you.
Use common sense when downloading. Research, check your system to make sure it can run the software, and use trusted anti-virus software. Good Luck.