A 12-year-old boy was rushed to hospital by air ambulance to Kings College Hospital after a six-vehicle crash on the coastbound A20 between Sidcup and the Swanley Interchange on Saturday night.
Kent Police is appealing for witnesses after the boy was left with serious abdominal injuries as well as head and chest injuries.
The boy was travelling with his family- another boy, a man and a woman- in a Renault Traffic minibus when the crash occured just after 8.30pm.
He is still being treated in hospital for his injuries, but they are not believed to be life-threatening.
The other three casualties were taken to Darent Valley Hospital in Dartford for more treatment.
The driver of a Renault Clio suffered concussion and minor head injuries.
Officers from the Kent Police Serious Collision Investigation Unit would now like to hear from anyone who saw the incident or witnessed any vehicles involved in the moments leading up to the collision.
Inspector Martin Stevens said: "This was a very serious incident which blocked the carriageway and led to the closure of the A20 from Crittals corner to the M25 junction.
"Thankfully there has been no loss of life, which could so easily have been the case.
"We would like to hear from anyone who witnessed the collision or who saw the vehicles being driven prior to the collision."
The coastbound carriageway remained closed by emergency services as the casualties were recovered and the scene was made safe.
Two KFRS fire engines, one London Fire Brigade fire engine, Kent Police, South East Coast Ambulance Service and the Kent Air Ambulance all attended the incident.
Anyone with information is asked to contact the witness appeal line on 01622 798538.