"We heard the noise of the impact and then my son just bolted and we heard the mother screaming," she said.
"When my son got to the scene the driver of the white car was trying to start his engine, so my son pulled the battery out of his car.
"He thought that was the quickest way to stop any sparks."
The incident had left the family shaken, she said.
"We are all still quite traumatised. The woman was screaming, 'My baby, my baby' and looking for her baby and then someone found him on the footpath. We saw the little boy before he had been tended to."
Police Serious Crash Unit were investigating the accident.
According to Auckland Transport data, more than 15,000 vehicles travelled on Pupuke Rd on average each day and the section of road where the crash happened was low risk, with no previous accidents reported there.