I don't know much about kids' sites, but for a 12-year-old who's interested in astronomy, I'd say that a lot of the "adult interested layperson" material would be appropriate and comprehensible. The Planetary Society (http://www.planetary.org/home/) just redesigned their site and there's a tremendous amount of good stuff there. NASA's Astronomy PIcture of the Day (http://antwrp.gsfc.nasa.gov/apod/astropix.html) is usually incredibly cool and the captions have links to associated material; it's a good place to start exploring. The Jet Propulsion Laboratory home page (http://www.jpl.nasa.gov/) is good for keeping up with planetary exploration, and the Cassini home page (http://saturn.jpl.nasa.gov/home/index.cfm) is frequently updated with interesting things.

Most of the government and nonprofit sites have "for kids" sections too, of varying quality.