Moments away from heart transplant surgery, 11-year-old Brandi Moore is lying on a gurney, crying so hard her body shakes. She grabs for the only mother she has ever known: her grandmother, Refugia Moore. "I love you, mi hija," Refugia, 68, assures her, using the Spanish words for "my daughter." "I'll be waiting here for you."

Refugia's brown eyes grow misty as Brandi is wheeled down the hall to the OR at Children's Hospital in Aurora, Colo. It's hard to watch this, Refugia says—even if she's seen it before. For the second time in less than a year, one of the three grandchildren she and her husband, Ray, 76, are raising is undergoing one of the most trying of surgeries: a heart transplant. Samantha, 16, Brandi's older sister, successfully had the operation last April; Gary, 15, will be next up in the near future, the family's doctors say. Refugia remembers the day the cardiologist delivered the diagnosis: "He said most often these children don't live very long, and there is no treatment—only a heart transplant. I heard everything he told me, but it didn't hit me. It was like I couldn't even cry."

All three of Refugia and Ray's grandkids suffer from a condition known as restrictive cardiomyopathy (RCM), a potentially fatal muscle disease that inhibits blood flow in the heart. "It occurs in about one in a million children, so to have all three kids be affected and need a transplant is extremely rare," says Shelley Miyamoto, their cardiologist at Children's Hospital. The kids' living arrangement is unusual too: Refugia, who has her Ph.D. in education, was a nursing supervisor in Las Cruces, N.Mex., a decade ago when the couple's son Gary Sr., 37, then in the grips of a drug problem, could no longer care for his three small children, ages 3 months to 5 years. (Their mother has no contact with them and has relinquished parental rights.) So Refugia and Ray, having raised a family once, decided to do it again, formally adopting the kids in 2000. "I've been Papa Daddy so long," says Ray, "it just comes natural."

For nearly a decade after the adoptions, the Moores—who moved their family to a suburb of Albuquerque, where Refugia, who is part Navajo, has family—enjoyed their second round of parenthood, despite bouts of exhaustion. Gary, a boisterous aspiring athlete, was the first to show signs of heart disease. "He was always short of breath," recalls his grandmother. Doctors at first thought that might be a side effect of medicine he took for ADHD. In 2006 more intensive tests led to the diagnosis of RCM. Refugia remembers the doctor's saying it was as if a part of Gary's heart was dying. Brandi, who shared some of his symptoms, was diagnosed next, then Samantha.

For the Moores, a tense game of waiting began. Their doctors explained the stakes: Samantha, Brandi and Gary would soon join the estimated 250 children under 18 waiting for a donor heart to save their lives—and every year, at least 50 don't make it. To use Ray's military insurance, from his days as an Army medic, the family had to drive hundreds of miles each way to Children's in Aurora. On one of her first visits early last year, Brandi asked a doctor whether Samantha, the first to be placed on the national waiting list, could die during the five-hour transplant operation. "The answer was yes," recalls Samantha. "But they said it's better to get a heart transplant, because if I don't, I will die."

Samantha's health was rapidly deteriorating. After just three weeks, word arrived early one morning that a new heart had been found for her. (The family knows no details about their donors.) Ray waited in New Mexico, where the younger kids remained in school, while Refugia kept vigil outside the OR. "It was such a shock to see her afterwards. She was so swollen," says Refugia. But Samantha's operation was a success. Following three months of recovery at a Ronald McDonald House near the hospital (and an episode when her body began to reject the transplanted heart, remedied by adjusting her medication), she got to go home to New Mexico in July.

After a breather of five months, Refugia was back, this time with Brandi, so weak she could barely climb four stairs. The cycle began again: 67 days on the waiting list, five hours of surgery and Refugia, praying in the waiting room. Samantha, by then a junior in high school and hoping to one day become a transplant social worker, came home from school the day of Brandi's operation, excited to hear her sister was recovering in the ICU. But when Gary, an eighth grader, got on the phone, he sobbed. "He's scared," says Refugia. "Separation is hard on the whole family."

Last week they reunited once again in New Mexico, but they are still wondering when Gary Jr. will end up on the transplant list. And like all transplant recipients, who must take drugs for the rest of their lives, the Moore kids live with the risk that their bodies will reject the new hearts. At their ranch-style house in Belen, N.Mex., Ray, who has handled all the cooking and cleaning for eight months, is happy to have his wife by his side. Refugia yearns for a holiday, any holiday, at home. "If we have to sell the house, the car, the dog, we'll do what we need to do," she says. "I can't imagine one of the kids falling dead." Throughout it all, she has been deeply grateful. "It's a gift," she says of the second chance her grandchildren are being given. "Someone died for them to live."