Owen Maughan, 27, and his father Patrick Maughan, 54, appeared at Maidstone Crown Court on Monday accused of the murder of Peter Maughan, four, on June 1 last year.
Prosecutor Richard Jory KC told Maidstone Crown Court that the pair had spent the day drinking in Rochester, Kent, before Owen Maughan began driving them home in a Ford Ranger pick-up truck.
Owen Maughan had about 12 bottles of beer and Patrick had about 13 pints, the jury heard, after they had been drinking in the Jolly Knight pub and City Wall wine bar.
During the journey on the A2 motorway, they saw Owen’s cousin Hayley travelling in a second pick-up truck with her partner Lovell Mahon, who was driving, and their children Peter, four, and his then one-year-old sister Annarica Maughan.
Mr Jory said: “For reasons that weren’t and aren’t clear, Owen and Patrick were or became very angry and seemingly expressed that anger towards the occupants of the vehicle.”
As they shouted abuse, Owen’s driving became erratic, the jury heard, with the pair chasing the family for several miles down the A2 motorway.
Hayley filmed video clips of them following them, leaving a voice note for her mother Erica in which she said she was afraid.
As both pick-up trucks left the A2 at Pepper Hill near Northfleet, Mr Jory said: “Whatever the reason Owen and Patrick were clearly, we would say, in a fury.
“Very angry at the occupants of Lovell’s vehicle.”
While on the slip road, Lovell said that there were children in the car and offered to go to the Maughans’ house to resolve the situation, Mr Jory said.
As the chase continued, Owen called Hayley’s brother Jason on the phone to say that he was going to ram the other truck, jurors were told.
He drove into the wrong lane and clipped the back of the car at 60mph, the court heard.
The family’s truck rolled over and Patrick and Owen Maughan drove off, pausing for Patrick to pull the registration plate off the truck.
Mr Jory told jurors the prosecution say the case is “slightly unusual” but is a case of murder, which is denied by the defendants, of Hill Rise in Darenth, Kent.
Owen Maughan has pleaded guilty to the manslaughter of Peter Maughan, and causing serious injury by dangerous driving and inflicting grievous bodily harm on Mr Mahon.
“It means Owen accepts he deliberately rammed the other vehicle, and he accepts he drove his vehicle dangerously,” Mr Jory said.
“He disputes he intended to cause any injury to any of the occupants.”
He added that Patrick Maughan disputes encouraging or assisting his son and that the main issue in the case will be their state of mind at the time of the incident.