Police were called on Sunday shortly after 4 p.m. to report a collision involving a car and three pedestrians at Eastcote Road, Ruislip.
The child was pronounced dead at the scene.
Another pedestrian, a man in his 30s, was taken to hospital and is in critical condition.
A man in his 40s has been arrested on suspicion of causing death by reckless driving, serious injuries while driving and driving under the influence of drugs.
He is currently in detention.
Metropolitan police officers – some of whom were off duty – and paramedics were on the scene.
The child’s family has been informed and is supported by specially trained agents.
A post mortem examination should take place.
Police are now calling on witnesses or anyone with dashcam or CCTV footage to come forward.
Met Police Detective Sergeant Sarah Donegan said, “Our hearts are with the parents of the child at what must be an incredibly difficult time.
“We launched an investigation to establish the circumstances of what happened and I ask anyone who was nearby at the time and who saw something, or who has dashcam or CCTV footage , introduce themselves and help us with our investigation.
Witnesses are asked to call 020 8246 9820 or 101 CAD ref 4892 / 25OCT20. You can also contact the independent charity Crimestoppers on 0800 555 111