Buy him/her a star atlas and get familiar with the constellations as soon as possible. Then step up to a pair of good binoculars to search for star clusters and look at the moon. You may also want to explore building your own telescope either using store-bought parts or from scratch. Consult Astronomy or Sky and Telescope magazines for more details.
You should get a subscription to Astronomy magazine and its Odyssey companion magazine for young children. Sky and Telescope is far too technical for an 8 year old. Odyssey is excellent for this age group and provides games, info and project information.
My father got me started in astronomy when I was 10 years old by taking me outside and telling me about the stars. He later let me use his binoculars, and by the following Christmas, my older brothers pitched in to buy me a small 3 inch reflecting telescope. It was a defining moment for my interest in astronomy because now I had my own telescope with which to see craters on the moon and the satellites of Jupiter. Eventually, my father and I worked together to build a 4 1/4 inch reflector and from then on, my destiny was pretty well sealed!