But five days out from surgery, Thrupp was told the operation had been pushed back to February.
The bombshell prompted Thrupp to contact the Herald on Sunday - and three hours after the newspaper started asking questions Auckland health board chief executive Garry Smith rang to tell her the surgery was back on.
The case is a rare insight into the decisions doctors across the country are being forced to make daily.
As the health dollar stretches thinner, "prioritisation" sees some patients passed over.
Thrupp said she was distraught to discover the surgery had been cancelled after a four-month wait.
"This just can't happen to people. The processes that are in place are wrong. You just can't play with people's lives like this.
"This is life and death for this child. If we don't get it done she is gone."
Demar's health problems stem from pneumococcal meningitis as a baby.
The disease left her with disabilities, brain damage and, as she grew, curvature of the spine which placed pressure on her internal organs.
Without surgery the spine is at risk of snapping. Even the impact of the spine on the organs would eventually kill her.
Starship has treated Demar successfully for years, and the surgery rebooked for Tuesday would see steel rods inserted along her spine to straighten it.
Thrupp had been warned the procedure was high-risk and had planned ahead should the worst occur.
"We've arranged the funeral. We know what she's wearing. We know all the risks. This is it for her. We have never changed our minds about it. This surgery is her only chance."
Thrupp had been told by the surgeon on a number of occasions he was ready to go ahead with the surgery.
He said the problem was with the team of specialists who would deal with Demar after the surgery.
"The aftercare team says they don't really want to deal with her because there's not much chance," Thrupp said she was told.
"Who has quality of life? Who decides who gets the chance and who doesn't?"
Thrupp said she was particularly distressed knowing delays in the surgery increased the damage her daughter was suffering.
"She's 11 years old. I don't want her to go out like that. I want her to retain her dignity."
Starship general manager Kay Hyman, speaking with Thrupp's permission, said Demar's case was assessed alongside others needing surgery at the same time.
Other cases had taken priority, leading to Demar's surgery being cancelled.
"There are a huge number of children and a small amount of resources, and we have to make the best decision we can.
"There is definitely a finite availability of resources of all sorts.
"There's a number of children and adults who might benefit and others who don't always.
"This would be one of those."
She said there were four surgeons in New Zealand capable of carrying out the surgery, but only two who were able to deal with severe cases like Demar's.
Starship did about 70 such procedures each year.
"Demar is not a child who will have a long life. It is a challenging clinical situation.
"There are regrettably many children who could benefit from surgery and a very small number of surgeons who can do the surgery.
"There is no doubt if we had more surgeons in New Zealand that would be better for New Zealand children.
"We are recruiting for a further surgeon so we will have another one. Realistically, that is not likely to be for some time."
Hyman said the rebooking of Demar's surgery became possible after staff examined the waiting list and moved patients to other days or doctors.
The family were contacted and told "after it became clear the impact the rescheduling might have on them".
