The 30-year-old woman died at the scene of the horrific crash. The six children, including a baby, were trapped and badly injured.
Emergency crews were called to the scene on Goomalling-Toodyay Road near Irish-Town Road at Wongamine, about two hours north-west of Perth, at 6.10am.
The children were freed by firefighters and four were flown to Perth for urgent treatment.
The other two children were taken by ambulance.
Their pet dog is also believed to be missing.
The youngest of the children is one and the oldest is 14.
Two of the children were in a critical condition on Wednesday night and the others were stable.
The 58-year-old cement truck driver managed to escape serious injuries.
The road was closed for the day as major crash detectives looked for clues to how the tragedy unfolded.