Invisible Child (1999)

When Annie (Rita Wilson) and Tim Beeman (Victor Garber) hire Gillian (Tushka Bergen) as a nanny, she accepts the job with the understanding that her most important responsibility is the middle child, Maggie --- an imaginary child created by Annie. Maggie is loved and acknowledged by everyone in the family. Tim and the eldest daughter, Doc (Mae Whitman), play into the charade for the sake of Annie's sanity. Unsteadily, Gillian begins to care for the imaginary Maggie, but she becomes concerned for the youngest child, Sam (David Dorfman), who believes that he has a sister he cannot see. She also worries about the responsibility Doc has taken upon herself to maintain her mother's fantasy. Torn between her love for the family and her uneasiness with the situation, Gillian contacts the authorities unbeknownst to the Beeman family. Soon after, Tim explains to Gillian why the family endures Annie's fantasy and why they care for Maggie. When Children Services comes to the Beeman home to question the family's situation, Gillian regrets her decision to contact the authorities.