Every Heart a Doorway (Wayward Children #1) by Seanan McGuire

Children have always disappeared under the right conditions; slipping through the shadows under a bed or at the back of a wardrobe, tumbling down rabbit holes and into old wells, and emerging somewhere...else. But magical lands have little need for used-up miracle children. Nancy tumbled once, but now she's back. The things she's experienced...they change a person. The children under Miss West's care understand all too well. And each of them is seeking a way back to their own fantasy world. But Nancy's arrival marks a change at the Home. There's a darkness just around each corner, and when tragedy strikes, it's up to Nancy and her new-found schoolmates to get to the heart of things.

Hardcover; 176 pages