Sen. Daniel Patrick Moynihan of New York -- a walking, talking exclamation point in a blue bow tie -- is possibly the only major Washington figure who's more interested in broken households and childhood poverty than in who's up or down in the Iran-contra hearings.

It is 7:45 on a steamy morning. As reporters drift in for breakfast the senator is already firing on all rhetorical cylinders. At noon on this day he will introduce his Family Security Act.

''A life-threatening situation!'' he exclaims -- "life-threatening to the great cities of the land. And don't you doubt it! Maybe it was this way in the 15th century,'' he says, but it shouldn't be this way now.

He is speaking of the collapse of the traditional two-parent family and what it is doing to children. America is the first modern society in which children, not the aged, are by far the largest group of the impoverished. He quotes Joyce Black, head of the Big Brothers/Big Sisters program: ''By the year 2000, about 47 percent of all the children in the United States will be sitting down to dinner with one parent. Let us hope they can pay for it themselves.''

If you're the sort whose blood count plunges uncontrollably at the mention of ''dependency'' or ''children having children,'' what you need is a bracing dose of Moynihan.

The terms and aims of the senator's new legislation, which he is sponsoring in behalf of the Governors' Conference, are guarded. His legislation to some extent reflects the disillusionment of late-20th century social science with the idea that social disorders are easy to diagnose or to treat legislatively.

Yet it is a horrifying fact that, with the collapse of traditional family structures and obligations, about one-third of American children are now living in poverty, or near it. It is an equally dispiriting fact that about half of all ''welfare mothers'' are in more or less permanent dependency on the dole. And it is a fact that this is far from what the architects of the original Social Security Act of 1935 had in mind.

Something has gone very wrong with the transmission of stable family structures in America. To the extent that public policy is to blame, something has to be done. That has been a Moynihan theme for upwards of a quarter-century, long before the thought was fashionable and, indeed, when to embrace it cost liberal vilification. The question is whether we can get beyond hand-wringing and sociological agnosticism and do something about it.

Moynihan's premise, absorbingly argued in his recent Godkin Lectures, ''Family and Nation,'' is that every nation has a family policy, calculated or not. It is reflected in scores of laws, regulations and policies. And what passes for a family policy in the United States is often a textbook study in unintended consequences, frequently disastrous ones.

Moynihan and others managed a modest rollback last year when the tax ''reform'' act raised dependent deductions. This excluded some 6 million or 7 million working poor from tax liability: a belated reversion, as he notes, to the tax policy of the 1930s.

Now he has zeroed in on the Aid to Families with Dependent Children clauses of the Social Security Act. These, he recalls, were originally meant as a widow's benefit. Today only 2 percent of the beneficiaries are widows. The rest are single-parent households whose heads are unwillingly jobless or persons (usually young unwed mothers) for whom the dole has become a way of life.

To the degree that encouraging dependent people to break out of dependency and into self-support is a ''conservative'' idea, Moynihan's bill is conservative. It is also conservative, as such things are conventionally judged, in strengthening the mechanisms by which local authorities can track down absentee fathers and deduct child support from their paychecks. From California, it borrows the idea of ''contracting'' between the welfare recipient and the state in a plan to get off the rolls and into self-support.

It would be a triumph, Moynihan says, if by the year 2000 casework could once again be a reality and social agencies could keep track of the needy and their problems. In this respect, the hopes are modest. But a remarkable political consensus is emerging in support of reform.

It isn't a glamorous subject. But the success or failure of the Family Security Act could be a telling sign of what kind of society America wants to be in the coming century.