If your kids enjoy being on the computer and are artistically inclined, why not put the two together? You can find free software online for them to start out with, or you could invest in a quality program that will take them the duration. The industry standard is Adobe Creative Suite which includes everything from web design software to print media software. This software is quite expensive, and would be daunting for most children to learn. There are however sites that tutor anyone in how to use these programs for a fee.
I would recommend starting out much simpler to let children get their feet wet with graphic design. Using a program like Kid Pix that is specifically designed for kids seems a much better way to go to me. In fact Kid Pix was a great favorite with my kids for quite a while. You never know, one of your kids just might decide to make a career out of graphic art!