This something I've been musing over for a while - what exactly defines a work as a 'children's book'? It's a nebulous term, which covers a lot of ground, from The Gruffalo and The Very Hungry Caterpillar, through Roald Dahl, to more nuanced works such as the Earthsea trilogy, His Dark Materials and The Book Thief.
I firmly believe that a good children's book should be thoroughly enjoyable to adults, with the exception of some books for very young children. Given that, I would define a children's book as 'a book which is best appreciated by a child, regardless of whatever merits it may have'. Note that there is nothing derogatory about the term, and it allows the work to have elements which are best appreciated by adults. If you accept the definition as worthwhile, it leads to a slightly different view of what is and isn't children's literature.
A famous example of a book which is often said to fall between children's and adult literature is The Lord of the Rings; under my definition, it is simply a children's book. I'd justify this by saying it's a work better experienced rather than reflected upon, and many of us become less able to immerse ourselves in fiction as we become older. I certainly haven't lost that ability, but my inner critic fires up more readily these days, often to the detriment of enjoyment. My preference is to read and watch - where possible - fairly passively, then think about the work later.
So, what are common hallmarks of children's literature? I'd say the primary emphasis is on storytelling over deep characterisation or complex themes, although the latter certainly aren't precluded. It's also generally more permissible - although often not desirable - to take more liberties the story in terms of coincidences, stereotyped characters and unrealistic behaviour, as the primary audience is likely to be less critical.
Other examples I'd put forward of what I consider to be children's books would be To Kill a Mockingbird, most of Stephen King's output and - applying the same principles to film - Star Wars.
I'm interested to hear what people think of the above. Do you think it's a useful way of thinking about the genre, and can you offer more examples of works that aren't usually thought of as being for children? Also, can people think of counter-examples which break the definition, such as a genuinely good children's book which is almost wholly unsuitable for adult readers?
(Caveat: as with almost any attempt with taxonomy, there will always be edge-cases which don't really fit any category.)