It could be they want her to be like them. Everyone in the other world has buttons on their eyes because they are dolls. The other mother is also evil and she sewed buttons on the other ghost children.
They are not real... except for the Other Mother. In the movie her lair is built like a trap door spider's. The Other Mother is evil and wishes to trap and feed on Coralline. *cue evil laugh* "Mwah ha ha a ha!"