Tomos's mother Louise Bunford tried to push her son clear of the Mitsubishi Warrior, which was towing a heavy water bowser on a trailer, as they jumped.
The boy was hit as the vehicle and the attached trailer jack-knifed down the slope, South Wales Central Coroner's Court heard.
rack marks show the truck skidded around 280 feet down the incline.
An inquest heard today that the incident happened as the family had been going to water their cows which lived on the land.
Tomos's father spoke of how his son had loved helping with the family farm, often telling classmates about feeding the animals, and how he had a young calf he walked around the yard on a lead.
Mr Bunford said the trip was something the family had done many times before and the boy's older brother Gethin and baby Clemmie were also in the truck.
Clemmie was saved by her mother during the incident and was passed to Gethin.
The father was emotional as he told Assistant Coroner Gavin Knox that as the vehicle had started to go out of control he feared it would fall off a 'sheer drop' into a disused quarry at the bottom of the slope.
Mr Bunford said: 'We all left to go over just like we did the week before.
'We didn't do anything different from any other day, we went the same route down an incline.
'As we were going down my son Gethin is in the passenger's seat, directly behind me would be our young baby, my wife Louise would have been in the centre and Tomos would have been to the left.