>>11
Seriously. Knife and Spyglass get into some pretty deep concepts. Not to mention that the entire series involves killing god and basically says "Religion is stupid, keeps you from living your life, and could end up getting you killed." Y'know that's not counting the scores of violent death that permeates the series that parents might object to.
Of course, that depends on whether Kids will get that far. I would think that only the most nerdy and book-loving kids will get past the parts with describing the Mulefa, even though I thought it was brilliant.