Heartbreaking details have emerged about the moments before two young siblings were killed in a devastating crash in Sydney's west, as their devastated mother speaks out and forgives the driver responsible.

Five-year-old Katherine Tha and her one-year-old brother Harry were killed on Wednesday afternoon after they were struck by a car while attempting to cross a suburban street in Cabramatta.

Their mother, Sok, 33, who was pushing Harry in a pram, was also hit and sustained minor injuries. She has since been released from hospital.

'We can't sleep. When I close my eyes, I see my kids,' Sok said.

She thanked the bystanders who rushed to help her.

'We love them so much,' Sok said.

Sok added: 'Why my kid die? Why, please, I hope it hit me, not my baby. I don't want anything. I want only my kids. My daughter, my son, my life. I feel my life is success but now empty. Nothing, nothing to me now. I can't do anything.'

'I can't describe it, last night I couldn't sleep,' their father, Vundy Tha, said. 'Even the driver, I forgive him already.

'Nothing can change, you know? Nothing can change. I couldn't get my kid back.'

The grieving parents lit candles, touched teddy bear tributes and placed gifts, including the children's favourite snacks, at the crash site on Thursday morning.

The young family were struck at the corner of Gilbert Street and Joseph Street after walking back to their car following the school pickup. Katherine's school is located just 200 metres away.

The Daily Mail understands Sok saw the driver approach the intersection, which has a 'Give Way' sign, and believed the car had stopped to let them cross.

Family friend Patrick Te told the Daily Mail that Sok was crossing the street, as she does 'every day'.

'She's seen him stop. She thought that he was going to give way and then he hit her, I guess, and then she was knocking on the window to tell him to stop, but he kept going and dragged the car, dragged across the road,' Mr Te said.

'She was saying that if he'd just stop, it'd probably would have been all right. It would have been not these massive injuries.'

'Sok was picking up Katherine from school. Katherine, who was in her first year of kindergarten, was excited to go home after just receiving an award,' he said.

'A mother not only lost a child - she lost both of her children, her only two children.

'Please kiss and hold your kids tight. Cherish every moment, because time with them is never promised.'

A neighbour, who became emotional as he placed a tribute at the crash site, said he raced from his home when he heard sirens.

He described the scene as the most horrific thing he had ever seen in his life.

'I witnessed the whole thing. This is horrific, I know the lady. She crosses the road here all the time.

'I'm just devastated, my 10-year-old son goes to the same school,' he told the Daily Mail.

'I saw them giving the children CPR on the road. It's just horrific. I took some flowers too to the school. I'm just devastated. This is the saddest day in Cabramatta. We've lost two beautiful young children.'

He also said the car had turned left from Gilbert Street onto busy Joseph Street when it collided with the family.

'Somehow the SUV went over the pram and over the toddler,' the man said, shaking his head.

'The gentleman that killed the children, I know him, he lives in this street. He was sitting there in complete shock and I was trying to pat him on the back and saying, 'Mate it's just an accident.''

'Five minutes before that child at school having a good time.'

The driver was arrested and taken to Liverpool Hospital for mandatory blood and urine testing. He was released without charge pending further investigations.

The driver was not known to police.

Fairfield City Police Acting Superintendent Timothy Calman said members of the public played a 'phenomenal' role in attempting to save the children.

'The community involved here this afternoon was … I can only say it's phenomenal,' he said.

'We've had a number of motorists that actually stopped, and collectively have moved the vehicle onto its side, tipped the car over, in order to get one of the children from underneath the vehicle.

'Other bystanders have assisted with CPR to assist the children and the children's mother.

'It's probably one of the most confronting scenes that police have come across involving young children,' he said.

A Transport for NSW spokesperson told the Daily Mail there is not a significant crash history at the intersection.

'Transport for NSW extends its deepest sympathy to the family and friends of those involved in yesterday’s tragic crash at the intersection of Gilbert Street and Cumberland Highway, Cabramatta. We also acknowledge and thank those who stopped to help at the scene,' the spokesperson said.

'Given there is an active police investigation we are unable to comment further at this stage.'

A NSW Department of Education spokesperson said support was being provided to affected staff and students of the school community.

'We are deeply saddened by the tragedy that has affected the local community and share their grief,' the spokesperson said.