"The two girls have been treated in hospital and are nearly ready for discharge. Their mother is in a stable condition and will be admitted.
"This is really positive news, all things considered, and would be good to share with your children if they are feeling concerned."
Teachers will be reassuring children in the morning and the Ministry of Education's trauma team will be available for counselling. Maindonald asked people to respect the family's privacy on social media.
Greetings to all our friends and families.
An update after the accident outside our school this afternoon. We...
Posted by Western Heights School on Monday, 13 November 2017
Witness Trudi Ashby was leaving school with her 8-year-old twins when the crash happened soon after 3pm near Sturges Road Kindergarten and Western Heights School on Sturges Rd in Henderson.
"I heard a crash and I turned and I saw a car cross the road into oncoming traffic," Ashby told the Herald.
The car hit a taxi and spun around. It mounted the footpath and hit the woman and two children before veering back toward the road and hitting the steel bars opposite the school near the pedestrian crossing.
The woman and children are thought to have used the crossing and were heading away from the school.
Ashby and two other people went to help the victims.
"We covered her with a jacket to keep her warm until the ambulance came," Ashby said.
The car was "going fast - faster than if you are pulling into a driveway", she said.
She later saw an older woman standing beside the car whom she thought was its driver.
"She looked to be in shock."
Ashby said there had been hail and heavy rain.
Principal Maindonald earlier confirmed to the Herald an elderly lady had been behind the wheel of the car.
He understood the incident had been a "complete accident" and said the most important thing was to allow both families privacy.
"We really feel for [the driver] and for the family that was hurt," he said.
"It was a really unfortunate accident. Both families are affected and we want to respect and support them, first and foremost. We are offering them our full support and want to ensure their privacy."
St John Ambulance service said it responded to the incident with four vehicles after being called at 3.14pm.
Sturges Rd was closed for two hours between Palomino Dr and Spence Rd but has now been reopened.