A spokeswoman from Waikato Hospital said on Monday morning all three patients were in a stable condition.
A nearby resident on SH27 told the Herald on Sunday he provided assistance at the crash scene after he went to check that his neighbour's cattle had not escaped.
"The car was upside down in the ditch. It had gone through some rails and landed in the drain," he said.
"I gave a bit of assistance while I was there. I helped get a couple of young kiddies out of the car and a gentleman. I then held up a cover until the fire brigade got there to keep them in the shade."
He was not aware of the cause of the crash, but said the children's grandparents were following in a separate vehicle when the crash happened.
The fatality was the tenth this year to date, just one behind the 11 people who had died on our roads at this time last year.