What makes a book a children's book

What makes a book a children's book (as opposed to a young adult book)? I've been reading Bridge to Terabitha, Wrinkle in Time, Whale Rider, Tuck Everlasting, Lois Lowry (The Giver, Gathering Blue, The Messenger and Number the Stars) and Philip Pullman. Length and language are certainly two criteria although I did not discern any dumbing down of the language in any of these books. It's hard to argue that Philip Pullman's Dark Materials trilogy (running almost 1000 pages in total) or even Harry Potter (which I have yet to get to) have been shortened for children. In fact the themes that Philip Pullman deals with – the original sin, the church and theology are not what can be considered topics that children think about and in the degree of violence in his books were almost too much (for me). And even though I don't consider myself religious, the anti-Church message was a little over the top. In fact, none of the afore mentioned books deal with what I would consider children's themes e.g. horses, pets, fairies, or anything in this vein. These books have the common thread that the main characters in the books are children – but Orson Scott Card's Ender's Game also have children as the main characters but is not considered a children's book. Is it all marketing? Jenny Downham's Before I Die is definitely a young adult book since it contains sex – and includes the incredible feat of a first person narrative who dies at the end. But what about the Twilight series – is it merely because the characters are young adults versus for instance Ann Rice's Vampire series? Again, is it all in marketing?