The following sites are all very good. Many have excellent pages for children explaining the geological principles clearly but in a fun and engaging way.

The BBC web sites are hard to beat:

www.bbc.co.uk/cavemen interactive games about cave men, the tools they used etc.

www.bbc.co.uk/dinosaurs

www.bbc.co.uk/beasts/fossilfun
How fossils are made

www.bbc.co.uk/education/rocks/stones.shtml shows the rock cycle very clearly.
Children will enjoy this site such which making a sedimentary sandwich, geology of the bathroom and graveyard rock walk.

www.enchantedlearning.com good for the rock cycle

www.ucmp.berkeley.edu fantastic site, very clear on all geological things

www.volcanolive.com make your own model of a volcano
live volcano cams