They used to beat up on Dnart Dennis at school, because, as 12-year-old Dnart put it about one of his schoolmates, "he thought he was going to catch it . . ."

"It" is leukemia. Dnart has "it.) Not catching, of course, but it took Dnart's friends some time to absorb this. They adjusted this way: "We agreed," says a friend, not to hit him that hard . . ."

The scrappy youngster (complete with putative friends) is one of three California children with leukemia who provide the focus for tonight's Abc News Cloeup, "Can't It Be Anyone Else?" (Channel 7 at 10).

On the surface, the childhood leukemia story is a positive one. Medical advances in the past decade outstrip those in any form of cancer. In most cases remissions can be obtained. In many they can be maintained for years. The one-time sentence of a few weeks or months has been stretched, for some, at least, to "indefinitely."

At what cost this is to the children and their families, the ABC documentary sets out to assess.

It is not an easy show to watch. In order to vanquish this killer of children, its would-be victims are subjected to pain, discomfort, humiliation -- the latter from chemotherapeutically induced baldness, the worst of the former from the inevitable bone-marrow tests.

The viewer is spared neither the anguish of parents faced with the loss of a child nor the suffering of the child being subjected to the excruciating procedures that will keep him alive.

Nothing escapes the relentles documentary eye, not Dnart's own bone marrow test, nor 12-year-old Diana Baroa's uncontrolled tears at the news that one is forthcoming and not 10-year-old Jimmy Bird's weeks-long ordeal of a bone marrow transplant (from his older brother).

John Korty ("Autubiography of Miss Jane Pittman" and "Who Are the Debolts?") was executive producer of the documentary. It is honest, even harsh, but not without a great deal of sensitivity. It is even, occasionally, funny. There is some spectacular use of uniquely TV techniques -- as in cutting back and forth between Dnart's bone marrow test and a juvenile psychodrama in which Dnart plays a doctor performing a bone marrow test . . .

The three major protagonists are gutsy and appealing. They are determined to survive and the sense of their determination is captured in the show. It is this which lends the documentary its verisimilitude and its strength. But its greatest strength was in the choice of the subjects and their families, and its greatest moments are in the participants' candor. For example, in a scene of Jim Bird answering questions from his classmates about his forthcoming operation:

"What if it doesn't work?" asks one. "I'm pretty sure it will work," he answers, almost casually, "it's worked for everybody else who's gotten it . . ."

Or Dnart Dennis' mother on how she sees his illness:

". . . as like having another baby . . . you live with it, nurse it. It's like having another mouth to feed . . . and expensive because it needs special food . . ."

Some of the children in the documentary have died since it was filmed. Of the three principals, Dnart is doing best and may be off chemotherapy in a year. Diana's blood count, which had been a problem, is stable enough to permit continued chemotherapy and although some cancer cells were discovered after Jim's bone marrow transplant, he seems to have responded to treatment and continues in remission.