Coraline

In the opening credits, hands made of sewing needles are seen recreating a button-eyed doll to resemble an 11-year-old girl named Coraline Jones before sending it out into a void. Coraline Jones, an adventurous 11-year old girl, and her family move from Pontiac, Michigan, to Ashland, Oregon, into the Pink Palace Apartments, a divided mansion. The other residents include retired actresses Ms. Spink and Forcible (who live in the basement) and eccentric Russian acrobat Mr. Bobinsky (who lives in the attic).
Coraline finds her new home boring, and her parents hardly pay any attention to her adventures, as they are always seemingly busy working on a garden catalogue. Coraline meets Wyborn "Wybie" Lovat, the grandson of the apartments' landlady, Mrs. Lovat, who has a missing twin sister. Coraline also meets a black cat that follows Wybie around. While exploring, she is given a doll that looks like her by Wybie and finds a small brick-sealed door which can only be unlocked by a small black key with a button for a handle. That night, Coraline follows a mouse through the door, where she discovers the bricks have been replaced by a long corridor. Coraline goes through it and finds herself in the Other World, which is another, smaller world. Its inhabitants have buttons instead of eyes. Her Other Mother and Other Father are more warm and attentive than her real parents, and they have dinner together before tucking her in in her Other Bedroom. To her dismay, Coraline wakes up in her real bedroom and is unable to convince her parents that she has actually visited the Other World.
Despite warnings from one of her neighbours, Coraline frequents the Other World. During her visits, she sees the garden that from sky looks like her face, she meets the Other Wybie, who is mute; the Other Mr. Bobinsky, who ringmasters a jumping mouse circus; and the Other Miss Spink and Forcible, who are young and perform theatrical acts. The Cat also goes into the Other World and is able to disappear and appear behind objects and can talk.
On the third visit, the Other Mother invites Coraline to stay in the Other World forever, on condition that she have buttons sewn over her eyes like everyone else. Horrified, Coraline tries to escape. The Cat tells her about the true nature of the Other Mother, as well as the Other World, and how "she'd just love something to eat". Coraline demands the Other Mother let her return to the real world, but she transforms and imprisons Coraline behind a mirror. There, she meets three ghost children, one of them Mrs. Lovat's missing twin sister; victims of the Other Mother, whom they refer to as the Beldam. They tell Coraline that the doll was sent to them to show the Other Mother what was wrong with their lives, then lured them away and tricked them into having buttons sewn over their eyes, eventually eating up their lives behind the mirror. In order to escape, they need their real eyes, which the Other Mother has hidden. Coraline promises to find their eyes moments before the Other Wybie grabs her. She finds out that his mouth has been stitched shut into a horrific grin by the Other Mother, but he helps her escape back to the real world. She also encounters the real Wybie again who asks for the doll but after hearing her stories of the doll and the Other World runs away believing she is crazy. The Cat tells her that her parents have been kidnapped by the Other Mother. She sees them write a message of "Help Us" through the mirror. Coraline burns the doll which was remade to look like her parents.
Coraline returns to the Other World to play a "game" with the Other Mother: to find her parents and the children's eyes. If she cannot, she will stay forever. Coraline manages to retrieve the children's eyes from the various "wonders" and the twisted inhabitants that the Other Mother made for her. She then confronts the Other Mother, who has reverted into her true form, an arachnoid witch with sewing needle-finger hands. One of the ghost children warns her that the Other Mother will never let her go, even if she wins. Coraline finds and frees her real parents from a snow globe and manages to trick the Other Mother and escape, blinding her (via the Cat) and severing her right hand. Coraline's parents return with no memory of being kidnapped.
The ghosts appear to Coraline in a dream to warn her that she is still in danger and that she must get rid of the key, or the Other Mother will find it. As Coraline prepares to drop the key into a well, she is attacked by the Other Mother's severed hand, but is saved by Wybie, who crushes it with a rock. Coraline throws the key and the pieces of the hand down the well. The next day, Coraline hosts a garden party for her neighbors and prepares to tell Mrs. Lovat about her adventures.

Question: Where does Coraline meet the 3 ghost children?
Answer: Behind a mirror