More potential resources...

These are gorgeous books, and I like the idea of the internet-linked ones...
Usborne History Make sure to peek inside a couple - really nice books. The Handbooks (about halfway down the page) look like they might someday be interesting to my kids. Time Travelers and the Visitor's Guide to... may be good too.

There are also these - Usborne Young Readers - several of which look like they'd be a fun supplement, like the ones on Gladiators, the Samurai, Crusaders, etc.

Of course, there are these, which I've only ever heard good things about. And for science these, which look interesting too.

I'm glad I have a few years to decide!