Have them watch any documentaries that they are interested in. A certain show may be too complicated for them, but they will ask questions and learn much more than the possibly could from the drivel that is actually aimed at children.
I used to always watch PBS with my family and pick up the books my father read. I rarely understood much at first, but I learned to think this way. My questions at first were silly, but after another year or so, I learned how to piece information together.
I am a firm believer that if you treat children like children they will stay children.