It was Thursday of a long week of nuclear politics. George Bush was on his way home from explaining our policy to Europeans. The president was talking about our options and zero options. The newspapers were full of words about armaments and disarmaments.

But in Brookline, Mass., Roberta Snow had just arrived at the first grade of the Driscoll School with a film crew. She was there, as she had been in other classrooms in the Boston area, to find out what children knew about the nuclear world, when they knew it, how they felt about it.

As an educator, the director of one high school program, "Decisions in the Nuclear Age," and principal investigator of a medical research program for the Study of Human Continuity, she is one of a number of people trying to understand how children grapple with the most ominous reality of their world.

As a grown-up, she is also one of a number of people--parents, teachers, friends--trying to figure out how we do, how we should, how we can, talk to kids about the nuclear world when we can hardly talk to each other about it.

Snow didn't embark on this quest as part of some no-nukes politics. She was led to it by students. Over a year ago, a class of teen-age students assigned to write letters to the Soviet Union had produced reams of copy that expressed their terror of nuclear warfare and their deep anxiety about the possibility that they wouldn't grow up.

For the past year, Snow, along with psychiatrist Eric Chivian, with funding from the International Physicians for the Prevention of Nuclear War, followed that lead and focused questions and cameras directly on this terror. They found it among tenth-graders, seventh-graders, fifth-graders and even third-graders.

The fear, so close to the surface that the students rarely needed more prompting than a single word--nuclear --to begin talking, runs through a disturbing videotape Snow and Chivian made, a tape that has been seen now in schools and meetings across the country. Its title comes from the words of an 11-year-old: "I Feel Like There's a Nuclear War Going on Inside Me."

The children in this film express the nuclear fear in different ways at different ages. The adolescents are angry and frustrated at the older generation. The 9- and 10-year-olds return to the theme of death and not growing up. The 7- and 8-year-olds focus on the fantasy that they would be left alone in a post-nuclear world.

But even the third-graders were aware of the possibility of nuclear war, far more aware than their parents knew--aware during the daytime and during their nightmares.

Snow and Chivian had come now to the first-grade curious to see if this perception went down to these children in small chairs, the children who wriggled in this semicircle, even to the boy whose shoes were tied on the wrong feet.

What they recorded this time were kids on the edge of understanding. Half of these children had heard of the word nuclear or, rather, overheard it: from older children, parents, the television news. Most had assimilated "the nuclear" into the roll call of monsters accumulated by every first-grade television fan and into the cast of characters who filled their nighttime anxieties.

One boy, to accompanying giggles, talked about staying up at night imagining a war in his closet. Another overheard his parents talking and worried that "my street might get blown up." When the class laughed again, a little girl said, "That's not funny . . . because it might come true."

In two more years, they probably will be as sophisticated as the third- graders with a concept of mass annihilation and an image of the mushroom cloud. In six years, they probably will experience feelings of denial and hopelessness. But on this day, emotions among small, small children were clear: when you heard the word nuclear, what does it make you think about? "Dying." How does it make you feel? "Very, very scared."

Snow and Chivian have been making videotapes in part, as Snow said, "to get adults to understand that kids really do know. A lot of parents don't accept this."

Many of us have--in fact, prefer--to believe that young children are unaware of the nuclear terrors of our world. To hear them struggling, as we struggle, with what it means to live in such a fragile world is enormously difficult.

We feel, as parents, called upon to protect our own, called upon to comfort them. Yet we are often equally frightened, equally impotent in the face of the "Unthinkable." We cannot turn on the light, point the flashlight upon our children's beds or into their closet and make their nuclear monster go away. We cannot make it all better.

But to deny this reality, to wrap this fear in family silence, is to leave children further abandoned in their own anxieties. "Sometimes," said a boy in this classroom, "it really scares me." He was six years old.