Alisha Konig and her husband Scott Phillips and four children including Alisha's seven-year-old son William Johns were in their early model Toyota Kluger at The Children's Hospital at Westmead on Tuesday when an out-of-control car hit them.

"We were rear-ended by a vehicle in which the driver was suffering a seizure. Their car accelerated into the back of our car, writing it off," Alisha said.

"We are currently stuck in Sydney without our car while our insurance works on assessing the car. It appears due to the damage it will be written off."

The family has already been reeling from the news that William, who was diagnosed with neuroblastoma when he was five, has relapsed and the cancer has returned.

A tumour on his kidney was confirmed last May and he has returned to The Children's Hospital at Westmead for surgery and further treatment, including chemotherapy and immunotherapy.

Now seven, William is in year one at Namadgi School.

The little boy is trying to be brave but it's not easy, as the family had been told if he was in remission for a year, the cancer was unlikely to return.

"He's not really coping, if I'm honest," Alisha said. "He's quite upset that he has go through this treatment again that makes him feel yucky. He doesn't really understand it."

William's treatment is expected to take at least six months.

Both Alisha and Scott can't work at the moment as they need to be in Sydney to be with William and to care for their other children. They would like to return to Canberra occasionally to check on their house but without a car that's made even more difficult.

Friends have established a Go Fund Me page for the family to survive financially during this time of crisis.

Donations can be made here.