I think you make a good point about the language Jason, but what sets the books slightly apart (and only slightly) from the so-called 'adult' books is that they deal with issues that a child might experience: responsibilty for ones actions, being an outsider, and dealing with feelings for other people. These are all things that we deal with as adults, but they start in childhood and are often more difficult to deal with then.
Tiffan in particular is a character that young people may identify with and even admire, because she has the same problems they do and she deals with them and becomes a stronger person for that. It gives children the message that things might be awful now, but you can get through them.
But they are also extraordinarily good stories and I certainly read them as mainsteam Discworld books. Terry's best books in my opinion are those that are multi-layered. There's a great story on top but as you think more about what happened in the story, you realise that there is a whole lot more beneath and these books are no exception. If you take the effort to peel off the layers you'll get rewarded for it.
I find the whole consept of 'children's books' a bit odd. Some children will (given the opportunity) read whatever is available. They'll certainly read the mainstream Discworld books and get something out of them. Even if they don't fully grasp the full story, they are accessible.
I love the Tiffany books. I met Sharlene due to Hat Full of Sky being put out too early in Tesco and my need to talk to someone about it. Sharlrnr had her book store then and had an advance copy. So we wrote to each other about the book, became friends and got married last year. So neither of us were put off by it being a 'children's book'.
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