There a few really good online educational games that budding science buffs will enjoy. I think National Geographic Kids has some really creative games — we like the interactive adventure games, especially Crittercam: Antarctic Adventure. You can check them out here
National Geographic Kids offers much more than games, so take some time to watch some of the great short videos.
NASA also has some games on a website called NASA Kids club and it has some interesting games. The NASA website is a great overall resource for learning everything about space travel and astronomy.