I think I've finally put my finger on what makes Pratchett's children's books read so much more darkly to me than his adult books. It's because the humor is missing. I don't think I'd say I Shall Wear Midnight is any darker, at root, than Night Watch. But the latter has that humor in it that's rooted so deeply in obscure puns, and pop culture references from decades if not centuries ago, and History Department in jokes. That's been the hallmark of his books ever since he got past the custard-pie period. And those don't go very well in a children's book, so they get left out and what's left is the serious core, a lot darker without all those little sparkles and rib-nudges.