Start kids on an empty Fluxbox, let them learn to add all the bells and whistles they can. Eye candy can motivate creativity pretty well, and Fluxbox can get pretty cool with the right addons. Some of the menu-making is almost like coding; it's almost an introduction to formal languages.

Actually, Openbox might be the thing these days. I'm not sure. Definitely not LXDE -- that's preconfigured.