A five-year-old girl had proudly shown her mother an award she received at school just moments before she and her baby brother were killed in a devastating crash in Sydney's south-west. 

Katherine, five, and her one-year-old brother Harry were killed when a vehicle struck them as they crossed a road with their mother, Sok, in Cabramatta on Wednesday afternoon. 

Friend Patrick Te has spoken out about the 'split second' in which the mother lost her children.

'Their mother, 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.

'As they were crossing the road together, heading back to the car, an oncoming vehicle suddenly struck both Katherine and the pram where Harry was sitting.

'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.'

This is a breaking story. More to come