I wrote an essay for an OU course on crossover fiction recently. Here's an excerpt:
Changes in society mean that children tend to grow up quicker than they did 100 or even 50 years ago, and so they become interested in older themes sooner. With these changes and with crossover fiction becoming more popular in recent years, this could mean that the way that children’s literature is classified has changed. Depending on the point of view, it could be seen as having expanded, as crossover and young adult fiction could be seen as part of children’s literature and have therefore expanded the age range of readers; or it could be seen to have shrunk children’s literature as by the time children reach secondary school they are reading crossover and YA fiction and no longer reading children’s books.
Crossover fiction, essentially, is something that's written for one age range that is read widely by that age range and another different age range. For example, Harry Potter was first marketed as a children's book, but gained a lot of adult readers and so is also now classed as crossover fiction. The later books in the series touched on more adult themes, so I wouldn't say they were children's books, but they were designed for teenagers - the age range which young adult fiction is aimed at. But because they were also still read by adults as well, those books were crossover and young adult.Certainly the classification of stories would now seem to be children’s stories for very young children, early readers, juniors, and then crossover, young adult and adult, rather than the age-specific range of children’s stories which used to be the classification system even 30 years ago. However, you could also argue that crossover fiction has been good for children’s literature by bringing it to a wider audience and making more books available to younger and older readers by classifying them as crossover.
An argument could be made that all YA fiction is crossover, because it will be read by younger and older readers than it is aimed for; but not all crossover fiction is YA. Confused yet?!