Police and paramedics were scrambled to the A1 at the junction with Ashley Lane after the boy, 2, was severely injured in a collision with a pick up truck.
The incident happened at around 2.10pm on Tuesday and the white truck, which had a trailer attached, stopped at the scene. No arrests have been made.
Investigators from the Roads and Transport Policing Command are investigating. They would like to speak with anyone who saw the collision or captured it on dash cam.
A police statement said: "We were called at about 14:10hrs on Tuesday, 7 November to reports of a white pick-up, with a trailer, in collision with a child on the A1 Great North Way close to the junction with Ashley Lane, NW4.
"Emergency services attended. The two-year-old boy was taken to hospital where he remains in a critical condition. His family are aware.
"The driver stopped at the scene. There have been no arrests.
"Detectives from the Roads and Transport Policing Command are investigating. They would like to speak with anyone who saw the collision or captured it on dash cam."
Anyone witnesses are asked to call the Serious Collision Investigation Unit on 020 8246 9820 or via 101 quoting reference CAD 3824/07Nov.
