Answer by taking a quote from the following article:

Although Price claimed his birth was in Shropshire he was actually born in London in Red Lion Square on the site of the South Place Ethical Society's Conway Hall. He was educated in New Cross, first at Waller Road Infants School and then Haberdashers' Aske's Hatcham Boys School. At 15, Price founded the Carlton Dramatic Society and wrote plays, including a drama, about his early experience with a poltergeist which he said took place at a haunted manor house in Shropshire. According to Richard Morris, in his recent biography Harry Price:

Price claimed to have attended a private seance on 15 December 1937 in which a small six-year-old girl called Rosalie appeared. Price wrote he controlled the room by placing starch powder over the floor, locking the door and taping the windows before the seance. However, the identity of the sitters, or the locality where the seance was held was not revealed due to the alleged request of the mother of the child. During the seance Price claimed a small girl emerged, she spoke and he took her pulse. Price was suspicious that the supposed spirit of the child was no different than a human being but after the seance had finished the starch powder was undisturbed and none of the seals had been removed on the window. Price was convinced no one had entered the room via door or window during the seance. Price's Fifty Years of Psychical Research (1939) describes his experiences at the sitting and includes a diagram of the seance room.  Eric Dingwall and Trevor Hall wrote the Rosalie seance was fictitious and Price had lied about the whole affair but had based some of the details on the description of the house from a sitting he attended at a much earlier time in Brockley, South London where he used to live. K. M. Goldney who had criticized Price over his investigation into Borley Rectory wrote after the morning of the Rosalie sitting she found Price "shaken to the core by his experience." Goldney believed Price had told the truth about the seance and informed the Two Worlds spiritualist weekly newspaper that she believed the Rosalie sitting to be genuine.  In 1985, Peter Underwood published a photograph of part of an anonymous letter that was sent to the SPR member David Cohen in the 1960s which claimed to be from a seance sitter who attended the seance. The letter confessed to having impersonated the Rosalie child in the sitting by the request of the father who had owed the mother of the child money. In 2017, Paul Adams published details of the location of the Rosalie seance and identities of the family involved.

Who was Rosalie?
Price claimed to have attended a private seance on 15 December 1937 in which a small six-year-old girl called Rosalie appeared.