Shortly after Christmas, 3-year-old Jennifer Sefton caught what appeared to be an interminable case of the flu.

Then, one morning in February, she couldn't walk. Five days later, her flu-like symptoms were diagnosed as childhood cancer -- neuroblastoma, a rare malignant tumor whose cells have invaded her bone marrow and left her with only a 10 percent chance of survival.

Since then, the lives of Karen, Robert, Elizabeth and Jennifer Sefton have become a blur of hospital vigils, chemotherapy, specialists, nausea, fear of infection, crash medical training, intravenous feedings and 50-mile drives from their Prince William County home to and from Children's Hospital.

Jennifer's tumor has grown larger and larger as her weight has dropped to only 30 pounds. Her last hope of survival, a bone marrow transplant from her 2-year-old sister, Elizabeth, had to be delayed indefinitely because she was too ill to survive the surgery.

Jennifer's tragedy has made her mother one of the first federal workers in the nation to be eligible for a novel government effort in "leave-sharing," in which other employes contribute sick leave or annual leave to fellow workers.

Up until February, Karen Sefton had been something of a model federal employe. Rising from a GS5 to a GS12 in six years as a computer programmer, she supervised junior programmers in the Army's giant Hoffman complex in Alexandria.

After the diagnosis, Karen Sefton requested a transfer from her supervisory job because "I couldn't be there, and it wasn't fair to the people who were being trained."

She tried working early in the morning while her husband worked late at night, alternating at Jennifer's bedside.

That routine lasted two weeks.

"I just couldn't work," she said. "I couldn't concentrate; emotionally I was a wreck. We thought we would have to sell our house."

Her heartsick supervisors advanced leave and sick pay under every possible generous interpretation of Army civilian regulations. It ran out last week.

In the meantime, the Army nominated her for a pilot program authorized by Congress to allow civil servants to share part of their leave with others who need it.

Except for the test, limited to three of the nation's 2.1 million federal workers, "leave-sharing" is prohibited.

The Office of Personnel Management, administering the pilot project, received 240 nominations. With few exceptions, each was sadder than the last.

Karen Sefton was one of the three selected to test the "desirability, feasibility and cost" of transferring leave. William J. Ault, an IRS examiner in Cincinnati, and Freda R. South, who works for the Department of Energy in Miamisburg, Ohio, are the others. Both have cancer.

The idea surfaced last year when Rep. William Lehman (D-Fla.) was touring an Internal Revenue Service office in Florida and employes asked him for authority to donate their leave to two colleagues. Shannon Chiles was dying of cancer, and her husband, Joe, wanted to stay home and take care of her and their two small children.

A provision was inserted into the $570 billion continuing resolution for fiscal 1987 authorizing the donations. About 5,000 hours of leave were contributed, according to the National Treasury Employees Union.

Shannon Chiles died on April 1, and Joe Chiles Jr. has transferred to an IRS office in Kentucky, where his family lives, and intends to return to work next month.

Claudia Cooley, associate director of OPM, said the cost of the pilot program will be negligible.

"Leave-sharing" actually saves the government money, according to Lawson Campbell, an employe-relations specialist familiar with the Sefton case. "If we hadn't done this, she would have resigned and we would have lost the services of someone who is a very good worker in an area where it is difficult to get good people to begin with," he said.

A government-wide program, however, would incur some direct costs because federal workers are entitled to 13 days of sick leave a year and take an average of 8.5 days. The rest is never used, and need not be budgeted. If leave could be given families with hardships, all 13 days would almost certainly be used for every federal employe.

Virtually all annual leave is taken, so its transfer costs little -- essentially bookkeeping costs. A bill introduced in May by Rep. Frank R. Wolf (R-Va.) would authorize leave-sharing, but limit it to annual leave.

The highly circumscribed pilot project allows the transfer of leave only within agencies, or subgroups of an agency. Leave donations must be made in writing and must occur only after an employe's own leave is exhausted. The test ends next April.

Forms to sign up to contribute to Karen Sefton are expected to be ready in two weeks. Only her fellow workers in the Hoffman complex will be asked. When a small group of them was surveyed informally about whether they would donate, 1,200 hours were volunteered.

Ironically, her husband, Robert, a water treatment plant operator for Arlington County, began receiving "leave-sharing" time last April when the family went to Philadelphia for the bone-marrow transplant. Leave-sharing has been allowed by the Arlington County government for more than 20 years.

A bone-marrow transplant, in which marrow would be extracted from Jennifer's sister, Elizabeth, requires 45 days of hospitalization.

Doctors at Children's Hospital of Philadelphia had to put off the operation because Jennifer was too sick for it. The tumor must be shrunk through chemotherapy before it can be safely removed and the transplant attempted.

Although they cannot be sure, the Philadelphia specialists told the Seftons that they think that Jennifer's cancer may have arisen from abnormal cells present at birth.

"She is a little scared," said Karen Sefton. "When we went to Philadelphia we said we were going to get the tumor out and get the transplant so she could get better."

Chemotheraphy has not worked as well as the doctors had hoped, Karen Sefton said. The tumor has grown around some blood vessels, has pressed on Jennifer's liver, enveloped her kidney and moved up into her chest.

"She can feel it. She can see it. She looks almost pregnant. Her belly button has almost disappeared. Once you have a relapse, your chances go down and down," Karen Sefton said.

Jennifer no longer eats. "We would try to make her eat and get mad at her when she wouldn't. But it's not her fault," her mother said. So Jennifer spends 12 hours each night being fed intravenously to try to get her weight back to normal.

Since the family returned from the heartbreaking trip to Philadelphia, Jennifer has spent only 10 days at home.

Her parents take turns staying with her night and day, driving the 50-mile trip each way to the hospital here and switching off taking care of Elizabeth.

"We have every hope she will recover," Karen Sefton said.