Kingston's great grand-aunt, of Opotiki, said Miss Mauheni was completely shattered over Kingston's death.
"She's still a mess, absolutely shattered," she told the Herald.
"She loved her baby - people were jealous of how much she loved her son. She was a really neat mum."
The woman, who asked not to be named, said Miss Mauheni's partner was also in complete shock over the loss of the couple's only child.
"He's not talking, he's totally unresponsive as well. Both mum and dad are completely shattered."
The woman said Kingston's death was a "freak accident".
"Things like this, they do happen - they're just horrible, horrific accidents.
"When she heard she'd ran over the toy motorbike, she went to get out to try and grab the motorbike out from under the wheel - she didn't see her son in front of the car.
"So she just pushed the car forward, Young mum 'shattered'
after toddler son's death
not knowing bubba was in front of it. Then when she pulled the bike out from under the wheel, the car went too far forward.
"But then when she went around and heard him scream, it was too late."
She said Kingston dashed in front of the car and when Miss Mauheni realised where he was, he had been hit.
"You know how kids can be really mischievous and fast - one minute they're in front of you and the next you're chasing them. That's what happened. He was probably trying to pop around the other side of the car to get in next to his mum because she takes him everywhere with her."
Kingston died two hours after he was taken to hospital.
"The doctors and nurses tried everything they could for him, but his little heart just gave way," the great grand-aunt said.
She described Kingston as mischievous, outgoing and interested in everything, and said he followed his parents everywhere.
"He was a little ray of sunshine ... He was a beautiful little boy, a honey."
She said Miss Mauheni and her partner were last night going to Opotiki, where both sides of the family could support them through their loss.
Eastern District police spokeswoman Kris McGehan said the investigation was on-going and it had not yet been established whether charges would be laid against Miss Mauheni.