Looking at some of the re-designed covers last week made me think of how covers get re-designed when books massively move readership. I'm thinking here of children's books that catch the adult imagination and are then given adult covers as well. This means the books can now easily sit in two places in the book store, and if an adult is reading one, they don't need to feel weird about reading kids books (and not to kids).
Harry Potter is of course the classic cross-over book.
This is probably more YA than true kids book.
Phillip Pulman's Dark Materials is an interesting one because although the main character is a child the author has said the book was never truly targeted at children.
The most recent popular cross-over success