Police were called at 12.56pm on Tuesday, 14 September following concerns for the welfare of the occupants of a residential address on Leyborne Avenue, W13.
Officers and the London Ambulance Service attended. Despite their efforts, a five-year-old girl was pronounced dead at the scene.
Her next of kin are have been informed. A post-mortem examination will be held in due course.
A 31-year-old woman was arrested at the scene on suspicion of murder. She has been taken into custody.
The girl and woman were known to each other.
Homicide detectives from the Met’s Specialist Crime Command are leading the investigation. .