Type the name of a color to make it so. Perfect for young ones:
http://stewd.io/colors

Yup. Just type a color name. (Or at least, one of the supported color names. I went for curation and simplicity here rather than infinite messiness. Say what you will.) As soon as a color name is typed the background color will change appropriately. So fullscreen it, hand it over to your young one, and give RED or BLUE a shot since they’re nice and short words with vibrant pay offs. Then explore from there. (If your child is more comfortable with capitals just engage your Caps Lock.)