One thing that surprises me whenever Discworld comes up is that nobody ever mentions Amazing Maurice and His Educated Rodents. I get that it doesn't really tie into any of the other story arcs, but it is indeed a Discworld book (the 28th). It's also one of the few that was intentionally written for children.

I read it with my 11 year old daughter and was expecting it to be fairly ho hum, but was absolutely blown away by the story and the satire.

Not only do I highly, highly recommend it for reading with children, it's absolutely a great read for adults.