In this episode, a young boy named Anthony Fremont has the power to control reality with his thoughts. He lives with his parents in an isolated home, and the episode features a little girl dressed in white named "Antonia" who is actually a toy doll that Anthony brings to life.
While the episode focuses on Anthony's powers and the dangers he poses, it's possible that the white dress and the doll aspect stuck with you, leading you to recall the "little girl" in a bedroom.