"We never thought there was a risk involved," said Horst, 33. "This was something preventable and this was something simple. This was a routine test."
The Ferreros hope to build a $300 million, 125-bed standalone children's hospital in Gainesville within five years. They estimate they must raise about 10 percent of the money - $30 million - with the remainder coming from bonds.
The couple, who are developers, believe if Sebastian had been treated at such a hospital, the cascade of mistakes that led to his death might have been avoided, or at least caught in time. The Institute of Medicine estimates that as many as 98,000 people die each year in U.S. hospitals due to medical injuries.
Although their son died after being treated at Shands University of Florida hospital and Shands AGH, both in Gainesville, the boy's parents hope the new hospital would be operated by Shands. It operates a children's hospital within the UF hospital.
"It will depend how involved the community gets and how committed UF and Shands are to make this a reality," said Debbie Joseph, the foundation's executive director.
The tragedy began when the Ferreros took Sebastian to the University of Florida Pediatric Outpatient Clinic for a routine growth hormone stimulation test suggested by his physician.
At 25 pounds, the boy was small for his age, and the doctor wanted to see if he might be a candidate for growth hormone therapy.
Sebastian's 32-year-old mother told him about the test, and said it would feel like a mosquito bite. She bought a portable Thomas the Train set to entertain him.
The test involved the infusion of the amino acid arginine into his veins. His physician prescribed a dose of 5.75 grams, but the prescription processed by the Shands Medical Plaza's outpatient pharmacy was 60 grams.
Hospital workers administered the dose, and did not realize the error even when the parents asked them to check their son, who developed a headache and appeared to be in extreme pain.
The parents took him home, but when he vomited and had seizures they brought him to the Shands AGH emergency room, where they waited four hours for the boy to be seen.
He was later sent to a pediatric intensive care unit, and a CT scan was misread because there was no pediatric radiologist on duty, the Ferreros said.
By the time he was transfered to the intensive care unit at Shands at the University of Florida, the boy was brain dead.
The Ferreros soon learned from their physician that the overdose was the cause of Sebastian's death.
Shands immediately admitted its errors caused Sebastian's death and its investigation uncovered another arginine overdose: The mother of a 5-year-old boy stopped a test when her son jumped off the couch screaming, "My brain is on fire."
It implemented a series of changes, including the establishment of an infusion center with specially trained nurses. It will order its own medication and fill its own prescriptions, said Dr. Mike Gold, the College of Medicine's senior associate dean for clinical affairs. The hospital also will increase its training and oversight.
Gold said building a new children's hospital would likely take several years, or even a decade. But he admires the Ferreros' efforts.
"I cannot imagine a worst tragedy than for a parent to lose a child," Gold said. "But they are taking what happened to Sebastian and trying to make the world a better place."