Queensland police assistant commissioner Brian Codd said it was a miracle the two children, aged 10 and 13, survived yesterday's tragedy at Queensland's Dreamworld theme park.
Four people were killed on the Thunder River Rapids Ride yesterday when a raft flipped.
"It's almost a miracle that anybody came out of that," Codd said. "If we're going to be thankful for anything I'm going to be thankful for that."
The children were thrown from the ride and then "managed to extricate themselves", he said.
Codd labelled the accident a "catastrophe".
He said four adults and two children were on the ride when the raft flipped and fell back on those in it, trapping people in machinery.
"The fortunate thing is that the two children managed to escape that disaster. Unfortunately the four adults did not."
He described the initial scene as "absolutely traumatic" for the children and has offered support to the families as well as members of the public who saw the horror unfold.
Police hoped to confirm the names of the deceased today.
A thorough investigation is now under way under the direction of the coroner, he said.
Codd said police owe it to those killed and their families "and the community of Queensland to get to the bottom of what caused this and to hold anyone accountable" with the aim of "ensuring that this can never happen again".
"These types of investigations are not quick and take a lot of time."
Specialist engineers will be at the park today, examining the ride's componentry and "the make-up of the whole ride itself", including policies, procedures and maintenance schedules.
Codd said the catastrophe affected not only the families, friends and witnesses but also emergency responders.
"This is having a deep and emotional effect on a lot of people. Obviously the community of the Gold Coast, the people associated with Dreamworld, primarily the friends and family of those who have been caught up in this horrible, horrible tragedy, but so too has this affected the first responders, police, [and] paramedics.
"Many of our officers have families and I don't think there's any of our officers who haven't enjoyed Dreamworld and that particular ride. We are providing maximum support in terms of our peer support."
He said the reality of what has happened didn't sink in until after officers had finished their work in the days or weeks ahead.
Codd confirmed that Dreamworld will now be closed for at least several days or for as long as they need to ensure that they carry out a thorough investigation.
The rafts involved in the accident have also been taken away for forensic examination, he said.
He labelled the accident a "human catastrophe".