A YOUNG boy killed in a "terrifying" crash on the M5 in Sydney's south has been described as "kind hearted" and "sensitive" who had his life ahead of him as his extended family reaches out for help to fund his funeral.

The 10-year-old died during a multi-vehicle pileup at Riverwood on Wednesday morning which crushed his family Mazda as he was being driven to school.

His 13-year-old sister, who was understood to be sitting in the front seat, suffered fractured ribs and head injuries and is stable at Sydney Children's Hospital.

Their mother, a 31-year-old woman, who was behind the wheel of the Mazda spent Wednesday afternoon in hospital before being released while two other women in separate cars also suffered minor injuries.

A 26-year-old truck driver was questioned by police before being charged early this morning with a string of offences including dangerous driving occasioning death and negligent driving occasioning death. He was refused bail to appear in court today.

A mobile phone was yesterday seized by police from the cabin of the truck.

"Police have taken possession of a mobile phone that was in the heavy vehicle and that will be subject to forensic analysis in the coming days," Traffic and Highway ­Pat­rol acting Assistant Commissioner Paul Glinn said.

The young boy's aunt has launched a crowd-funding Facebook page to help raise money for his funeral, describing him as a "very sensitive" soul who adored animals.

"He was getting to an age where he was figuring himself out a bit more," she said.

"The last thing my sister needs is to worry about how she is going to afford a funeral, please contribute as much as you can or share it," she said.

The money will go to cover funeral costs, medical bills and daily expenses to help her sister recover.

She said she will be spending the day with her "understandably distraught" sister and all she can think about is how "messed up" the entire situation is.

Tributes have flowed in on social media for the young boy and his family.