The Mudgee woman says she won't know for seven years whether her daughter Summer-Rose, who turns three this week, will fully recover from the severe whiplash that caused the young girl's neck to break in three places.

The trauma has given Summer's father terrible nightmares, although he wasn't to blame for the accident and the forward-facing car seat had been properly fitted, Ms Brown said. She now lives in fear, imagining accidents happening, and hearing the sound of the crash over and over again.

Even Summer, who has a small scar on her forehead to show for the months in hospital, has changed her behaviour.

"Careful, careful, you got to be careful," Summer warns her family and friends.

To protect her neck, she can't bounce on a trampoline, play sport or go anywhere where other children play vigorously.

Ms Brown's post on Facebook last year urging parents to keep their children rear-facing in vehicles struck a chord with those lobbying for Australian laws to be changed to make it compulsory to keep youngsters facing the rear for much longer. Sweden recommends children be kept rear-facing until the age of four.

Ms Brown had always been unsure about turning her children around, but that changed after the crash.

"I will forever rearward-face my babies as long as I possibly can. Don't make the same mistake as I did. It could cost you your baby's life," she urge friends.

Summer's sister Emelia, now 23 months old, was in exactly the same restraint but it was reverse-facing when the family's car flipped over. Emelia suffered a small bruise to the shoulder.

In Australia, babies up to six months old must be restrained in a rear-facing seat, shown to reduce their risk of injuries by as much as 90 per cent. Children from six months to four years of age must use either a rearward-facing child seat or a forward-facing child seat with a built-in harness.

Experts who wrote the national guidelines recommend children be kept rear-facing for as long as possible to stop heads and necks from moving suddenly. Some newer restraints can be used rear-facing for children up to three years.

Young children have heavy heads, with thin soft bones and large brains, making them much more likely to suffer head injuries in a collision. These big heads are perched on on a slender neck which is generally not developed enough to "dampen violent movement", finds a seminal American study by the University of Michigan.

At birth, brains are around a quarter of their adult size, though babies are only 5 per cent of adult weight, according to the study.

"In a collision, an unrestrained child because of his large head and high centre of gravity, would 'lead with his head'," wrote US expert Donald Huelke of the University of Michigan Transportation Research Institute.

Experts said they could not comment on individual crashes like the Browns.

Professor Lynne Bilston from NeuRA, a medical research organisation which helped develop the national child restraint guidelines, said experts recommend that young children stay rear-facing for as long as they can. Evidence shows this reduces their risk of serious injury by about 80 to 90 per cent.

But so far there was a lack of evidence to support a change in the law to make it mandatory for older children. There were too few cases, and the outcome of each depended on the type of restraint used and exactly how the car was damaged in the crash. 

This story was originally published by the Sydney Morning Herald.