I've always thought it was more cute than sinister.
OK, since I've generally been good at figuring out magic tricks, I'll give my spin on how this might be done without the use of demon* or ghost children:
First, if you'll notice how the girl is motioning towards the right mirror, as if to point out the aberrant reflection. This indicates to me that she's in on the trick, which is all it is.
I think it's done with identical twins.
The twin shown in the rightmost mirror (notice the angle) would be sitting to the left of the visible girl, just out of camera range (A matter of only a couple of feet would suffice.)
Since there is no sound, I think their motions are synchronized by the direction of the camera operator or some other person.
That's how I think it's done.
*Some people are of the opinion that all children are demon children. Evidence seems to be mounting.