World that Never Was - Arthur St John Adcock

Olive, age seven and a half, and her brother Tony, who is one year younger, live in London. Because their mother has been sent away ill, they are being raised by their beautiful but unpleasant Aunt Prue, their dad, who is a businessman too busy to see them, and their loving but homely nurse Becky. One night, a mysterious appearance by their mother leads them to a great adventure. Even though Becky has warned them not to, they decide to go out at night to see what the city is like when everyone else is home sleeping. At the stroke of midnight, magical things happen. - Summary by Verla Viera