Bunny Lake is Missing
(1965)

Bunny Lake (Suky Appleby) is a real child and still alive. Steven Lake (Keir Dullea) kidnapped her from school, drugged her and hid her in the trunk of his car.

Ann (Carol Lynley) breaks out of the hospital and finds Steven about to strangle Bunny. Ann distracts him, reassuring him he didn't do anything wrong. Steven turns on her and makes it clear he resents Bunny's father and Bunny herself for monopolizing Ann's love and attention. A panicked Ann suggests they play hide and seek, and keeps Steven occupied with a series of their childhood games until Newhouse and other officers arrive.

They reveal Steven lied about the date Ann and Bunny came to England, arrest him, and Ann is reunited with her daughter. 

Thanks Emily!