Heather Bixler wishes she could undo the moment she's relived countless times: She was leaving her New York apartment with her 4-year-old daughter and infant son, who was in a baby carriage. It was May 2, 2003, and they were going to rent a movie.

The doorman, perhaps just to play around, picked up the stroller and held it almost vertical. Sean, the baby, fell out. His head bashed against the marble stair.

Sean's skull was fractured, and he was bleeding internally. The incident left him with scar tissue in his brain.

Two years ago, the seizures started. So did the never-ending medical expenses.

The Bixler family is just one example of how a child's chronic illness can strain a family emotionally and financially -- and children represent the fastest growing health care spending group in America, according to a new report.

Bixler is a professional violinist, her husband teaches music at a local college; they now live in Bowling Green, Ohio. Their children are covered under his insurance, but the coverage doesn't include everything Sean needs to be healthy -- they could end up paying $50,000 or more this year alone, depending on what their insurance ends up covering.

And Sean's mother can't work because someone needs to be with Sean at all times. She estimates that between 30% to 50% of the family income goes toward care for Sean; most of the costs have been out-of-pocket.

"I have put my life on hold," says Bixler, 47. "I've depleted my entire savings account, pretty much," she said.

When Sean started having seizures, Bixler took him to a neurologist, who diagnosed epilepsy. Insurance covered 80% of the procedures required to make that determination, but that still left the Bixlers with about $1,500 to pay out of their own pockets.

The boy's condition worsened last fall, when Bixler started pursuing a college degree in Memphis that required her to leave their Ohio home for half the week.

"He started having seizures all the time, sometimes six a day, gran mal seizures that would last a couple minutes, and would be so bad that a couple times I called the ambulance because he was turning blue and stopped breathing," Bixler said.

If someone wasn't right by Sean's side during a seizure, he would fall. Once, he was holding a glass in his hand, it broke, and he fell into the shards, slashing the side of his head.

"He had to be watched constantly. And if he wasn't being watched, like if I had to take a shower, I would make him sit in a cushioned armchair and tell him, 'You can't move,'" she said. "It was more work than when he was a baby."

They tried another neurologist. Different medication. Alternative medicines.

As the seizures got worse, so did the child's ability to communicate. Sean had been an "unusually smart" until recently; he could tell you names of complicated dinosaurs, Bixler said. He was talented at art and would draw so much that Bixler couldn't keep all of his works. But in January, Sean's teacher told Bixler that Sean had failed a spelling test.

In February, he couldn't read his own birthday cards. He lost the ability to answer basic questions -- even "What's your name?" Sometimes he couldn't even talk beyond "hi Mom" and "I love you."

A $31,000 visit to the Cleveland Clinic determined that the seizures are connected to that scar tissue that formed after Sean hit his head on the marble stair. Bixler's insurance company initially told her that that expenditure wasn't necessary; Bixler fought back, and is waiting to see how much of it will be covered. The family is fund-raising for Sean through the website giveforward.com.

She's now paying about $100 per week more for groceries because she has put her son on the ketogenic diet -- a strict high-fat, low-carbohydrate regimen -- that has helped reduce seizures for other children with epilepsy. On Tuesday, Sean's first day on the diet, he suffered a couple of seizures, but as of Friday he hadn't had any others. And he actually was able to read a page of a book -- although slowly and painstakingly.

"I'm trying to work with him to try to get him to read again, and then eventually we'll try to do some math," Bixler said.

Families like the Bixlers are always paying attention to the math of adding up medical bills.

Health care expenses growing fastest among children

A report released Monday from the Health Care Cost Institute, details how out-of-pocket health care expenses are on the rise, and spending in general is growing fastest among Americans 18 and under.

The institute is an independent nonprofit research organization that partnered with four major insurance companies (Aetna, Kaiser, United and Humana) to analyze 3 billion insurance claims of people with group employer-sponsored health insurance.

The study said consumers' out-of-pocket expenses rose 7% from 2009 to 2010, according to the institute. For insurers, costs only rose 2.6% during that time period.

Per person under 65, the average annual spending on health care was $4,255 -- that's a combination of what people and their insurance companies paid.