The narrative technique in Coraline is third person limited, which means that a narrator tells us the story, focusing mostly on one character. In this particular case, an adult narrator tells us Coraline's story.
This is a pretty common narrative technique for books that star children. Having an adult narrator tell us a child's story can give us a little more depth; sometimes adults can see the big picture a little more than kids. Also, it's usually just easier for an adult to write from an adult's point of view!
Gaiman's use of third person narration also gives us an important outsider perspective on Coraline. Coraline is often the only character on the page; she doesn't get to have a ton of conversations with other people (except the cat, of course). Having the story narrated in the third person gives us that outsider perspective we'd otherwise be missing, without losing insight into Coraline's inner thoughts.