Encyclopedia Britanica Online
The other great thing about this site is the children can build up the information they find by saving it as a project. As they find information they can save it to a project website, give the project a name and a password. Children can go back to the project and add more at a later stage.
New Zealand public schools can get on using the EPIC link from TKI and their digistore passwords. Other schools I'm not so sure, I assume you will need to aquire a licence from the site itself.