The black button eyes of the other mother are probably the most iconic or well-known image to come out of Coraline.
They were looking at her with their black button eyes. Or at least she thought they were looking at her. She couldn't be sure. (4.128)
These eyes are definitely creepy. The black button eyes let us know that we're in the other world and highlight how disturbing it is. Buttons are a seemingly harmless object, but in the other world, they become sinister and threatening. In fact, turning nice things into threatening things seems to be one of the main functions of the other world and the beldam herself.
These buttons essentially act like masks. Coraline can't tell if her other parents are watching her and she can't get any clues through their eyes as to what they're thinking. Because we see so much through people's eyes, these buttons hide the humanness of the people wearing them (if they're even human at all), preventing us from really knowing a person. For this reason, Coraline's other mother could never really be known, or loved, by the children she kidnaps.