CHILDREN'S HOSPITAL. By Peggy Anderson. Harper & Row. 528 pp. $18.95.

WHEN CHILDREN are sick, the world is turned upside down. The pastimes of childhood -- running, playing, learning, imagining -- are cast off, and instead the children grow somber and wise. Their parents, no longer concerned with school grades and skinned knees, become consumed with king- sized worries, incensed with God, terrified of a distorted future in which their child's death precedes their own.

How can anyone deal with such catastrophe? Author Peggy Anderson, in her ambitious book Children's Hospital, spent five years trying to find out. "My intention from the start was to explore the responses of those involved when a child is struck down by serious illness or injury," she writes in the book's preface: "to discover how -- or wheththe child, the family, and the hospital staff members attempting to save or cure the child are able to tolerate the intolerable."

Anderson chose to write about six valiant children and the people who love and care for them -- their mothers and fathers, their nurses, their doctors, their therapists. She gives us Candy, age 6, born with a facial deformity being corrected at Children's Hospital by state-of-the-art plastic surgery; Jody, born with so many internal anomalies that he has been here for the entire 18 months of his life just getting his insides rearranged; Freddy, a 9-year-old in a coma after a near- fatal car crash; Gina, who is 8 and has leukemia; Brandon, born three months premature and suffering all the risks and problems that premies endure: and Mark, a 15-year-old with cystic fibrosis.

Interspersed with day-by-day accounts of these youngsters' hospitalizations at the Children's Hospital of Philadelphia, where the action takes place, are descriptions of the changing pattern of childhood illnesses, accidents, and mortality in America; intimate accounts of surgery at Children's; and discussions of hospital policies regarding methods of payment, initiation of life-sustaining therapies, even meals (hot dogs are offered at every dinner). We are privy to nurse-patient repartee, the involvement of child welfare authorities and pastoral counselors, the musings of physicians at home or in a darkened hospital hallway.

Anderson's researching skills are impressive, and her writing style, although often awkward or riddled with clich,es, is appropriately low-key. She chooses to include as many details as possible, and the details provide all the emotion the book needs. Suspense runs high: Will Freddy be brain-damaged? Will Gina relapse? Will Brandon die? And we are in suspense because we do indeed care about these children.

The book really belongs to Mark Price, the skinny 15-year-old with cystic fibrosis, a genetic disease in which the body's secretions -- sweat, saliva, and mucous -- are so thick and gummy that they interfere with breathing and digesting. Only half of all cystics live to the age of 20. Mark has horn- rimmed glasses, blue saucer-shaped eyes, and an outgoing, inquisitive, and stunningly earnest personality. We meet him the night one of his lungs collapses, and we know -- as he knows -- that he will die soon. Mark is the kind of articulate, wise-beyond-his-years boy who consults on his case with his doctors and who actually attends conferences at the hospital on cystic fibrosis. His nurses adore him, and Anderson quite obviously does, too. He is so engaging that he sustains the whole book. While Candy is whining or Gina is being silly, we keep reading because we want to get back to the Adolescent floor to see how Mark is doing.

Only the hardest heart can fail to respond to Mark. "How long do you think I'm going to live?" he startles his parents by asking one evening. "Well Mark," says his mother once she has regained her composure, "I used to think you'd live a long time. To forty, maybe. But you've been sick so much lately that several times in recent weeks I would have told you a few days. Today I feel optimistic. I would say maybe early twenties. Twenty- five."

IT IS MARK, a boy of 15, who must guide his parents through the terrain of this own death at an early age. His goal, he tells them, is to live long enough to finish high school and to learn to drive -- but he has plans for after he dies, too. "When I do die," he says simply, "I don't want you guys to miss me so much that you'll ruin your lives. . . . I want you to go on and be happy. Just try to have happy memories of me." Mark has one request: to have an autopsy, because "how else can the doctors learn about this terrible disease?" He is the only child in his doctors' experience ever to request one.

We learn about Mark's hard life in bits and pieces. He moved to a new town only the previous summer (his seven-week hospitalization begins just after Thanksgiving). Unable to bike through the neighborhood to make friends, he had stayed at home with the dog. But he was happy to start 10th grade in September, even though his illness make going to school a challenge. "To cushion his thin buttocks against the hard wood seats," Anderson writes, Mark "had worn an extra set of underpants. His mother's suggestion that he take a pillow had exasperated him. 'Mom! You can't carry a pillow around!' Because his bowel movements were so foul- smelling, he had used the toilet in the school nurse's office. His brother had helped him from class to class . . . (After school) Mark had insisted on walking home. Or, more precisely, on dragging himself up the hill."

Mark's bravery in the face of excruciating pain is enormous. His optimism in the face of defeat is wondrous. And his maturity about confronting chronic illness is impressive. "You know," he tells his parents as he unwraps a before-dinner cupcake, "you guys are so worried about me that you're letting me get away with stuff you never let me get away with before. Like you always made me eat my wholesome food before I could eat junk food. Now you're happy to see me eat any old crap. That's not good. Even though I'm sick, you've got to keep me in line." His mother good-naturedly takes the lecture to heart and tells Mark to put the cupcakes away until after dinner.

When Children's Hospital focuses on Mark, it is extraordinary. Peggy Anderson quite obviously fell in love with the boy and his family -- even with his doctors and nurses. But Anderson tried to do too much. She gave us Mark, and she tried to give us five more children as well. In enlarging the scope of her work, she ultimately diluted it.

The book could have used a mean-spirited editor, someone who would say to Anderson, "This girl talks in exclamation points; get rid of her," or, "This boy's progress is too slow; cut him." The inclusion of all these patients does show us the variety of illnesses children face and the range of methods parents use to cope with this most awesome of family tragedies. But does it have to show us in such detail? I kept wanting to get back to Mark. The other kids and their disparate problems were too much in the way.

Children's Hsopital is a difficult book to read, especially because the children have been struck down by seemingly random events that can happen to any of our own children. But it is an impressive reportorial feat, despite some awkward turns of phrase, and an instructive look into the workings of one hospital. And it helps us understand the peculiarities of illness in childhood, from the perspectives of the children and the people who love them.

"Staff members at Children's Hospital are sometimes asked why they or anyone would choose to work with sick children," Anderson writes. "Hearing the answers, one almost wonders why anyone elects to work with sick adults." As one physician put it: "Kids are innocent. You always feel you're making a contribution if you help one. You help a rapist who's been shot as you would anyone else, but you certainly have reservations. You don't have reservations with a child."