UPDATE: A six-year-old girl involved in the crash on the Bruce Hwy at Torbanlea has been transferred to the Royal Brisbane Hospital for specialist care.
A Queensland Health spokesman said she was in a stable condition.
He said the girl's twin was admitted to Hervey Bay Hospital overnight and a 9-year-old girl and their grandfather were discharged on Wednesday.
EARLIER: Seven people are lucky to be alive, after escaping an horrific Bruce Hwy crash near Torbanlea on Wednesday.
Police investigations indicate a four-wheel drive mobile home heading north on the highway veered onto the southbound lane, rolling down an embankment and crushing a ute.
Simon Smith, of Torbanlea, who was driving home with his father, witnessed the accident.
"We were driving north and it was 200m in front of us," he said.
"We got out and ran straight over to it."
Mr Smith said about 15 passers-by stopped to help before emergency services arrived.
Together they worked to help free the young children trapped in the back of the ute.
"All us people got the girls out," he said.
He said the scariest part was seeing a young girl covered in blood.
Mr Smith said he thought the mobile home may have skidded across the highway after stopping suddenly to avoid running into another car slowing for nearby roadworks.
"We saw the impact of the hit, it flipped and landed on top of the Colorado (ute)."
Four people, including two children, were injured in the accident and taken to Hervey Bay Hospital for treatment.
A third child in the ute escaped injury and the two occupants of the mobile home were also unhurt.
The crash happened around 11am, between the BP Service Station and the Torbanlea turn-off, between Maryborough and Childers.
Police and fire service officers attended and part of the Bruce Hwy was closed.
The forensic crash unit is investigating.