"He can't believe what happened; none of us can," the relative said.
The man was last night making his way home to Greymouth, where family members and dozens of people from the community had gathered at the Grey Base Hospital to offer their support.
The 4-year-old had bruises and scratches "but he's fine", the relative said.
The family did not know how the crash had occurred.
"They went out somewhere and they were on their way back and that's when the accident happened. That's all I know."
Sergeant Dave Cross of the highway patrol unit said it was wet and the vehicle was travelling downhill into a bend.
West Coast police area commander Inspector John Canning said the victims were from Cobden.
Grey District Mayor Tony Kokshoorn said locals would do their best to support the family.
- additional reporting Greymouth Star