Have you seen DeweyDigger yet?

By clicking on one of the Dewey classifications, you are then presented with a range of topics that would be found under that classification.

Clicking on one of those you are then presented with a range of options to dig further.

Beware that some of these options include advertisements and one that I saw was for gambling. However, the initial stages of the site could be very useful for teaching the concept of Dewey. Worth a look.