What's next on the nation's political agenda? It's not hard to spot and, for once, it's not in line with the goals of the Reagan administration. The next item is what to do about America's poor children. Daniel Patrick Moynihan, ahead of the curve as usual, made it the subject of his 1985 Godkin lectures, published this month as "Family and Nation." Leon Dash's brilliant Post series examined in depth and over time the young poor black women who became teen-age mothers -- a subject also addressed in Bill Moyers' moving Jan. 25 CBS special.

Ronald Reagan in his State of the Union echoed them. "In the welfare culture, the breakdown of the family, the most basic support system, has reached crisis proportions -- in female and child poverty, child abandonment, horrible crime and deteriorating schools." The Democrats, lampooned when they put a Reaganish gloss on their rhetoric, heard Reagan say words that could have come from Pat Schroeder or Marian Wright Edelman. True, the president is putting off action pending a council report after the November elections. But that was what he did in 1984 on tax reform, which ended up near the top of the national agenda in 1985 and 1986. The extraordinary thing about that issue is that support came not from constituencies but logic. Tax rates have been flattened by preferences and tax avoidance, to the point of undermining confidence in the whole tax system; the response, from Bradley-Gephardt to Kemp-Kasten to Treasury II and Rostenkowski, were rate-flattening, preference-cutting bills. Similarly, the problem pointed to by Moynihan, Dash, Moyers and Reagan activates no constituencies. The impetus for action comes from logic. The problem can be put, with only a little exaggeration, this simply. We are a nation of rich adults and poor children. We need to do something about that.

The problem can be described demographically. In the baby boom years most people had lots of kids, and the incomes of kids' households matched the income of adults' households. Today, poor people have a lot more kids than the rich or middle income, and the income of kids' households is well below that of adults. The problem is especially glaring among the highly visible minority of blacks who live in an underclass society in which young males routinely father children and then just as routinely fail utterly to meet their responsibilities to them.

At first the problem seems intractable. But the directions society needs to go will become clear as Americans think about it. One direction is to provide government aid. The other is to buttress traditional moral standards.

Government aid is needed to jog young mothers out of the slough of welfare hopelessness, as Massachusetts Gov. Michael Dukakis' employment and training program has. It has trained and gotten jobs for 23,000 welfare recipients, partly because it keeps them eligible for Medicaid and gives them free child care for a year. These expensive but temporary services help mothers get hooked on work and help them do well enough to afford health insurance and child care. Dukakis says: "We have found the answer to the welfare problem." Maybe, maybe not. But what he has proven is that rather small amounts of government money, intelligently spent, can make a difference -- and even save money.

Reagan may expect his domestic council to recommend something like Charles Murray's solution -- cut all welfare benefits except unemployment insurance. That would be conveniently inexpensive, but it would not survive for a minute the inspection it would receive in the political process. Politicians of both parties understand instinctively that there's no free lunch. Reagan has put on the national agenda a question to which answers come more naturally to Democrats than Republicans.

The second direction we must take, buttressing traditional moral standards, sounds more Reaganish. Yet even here the Democrats are singing his tune. Behind much of the backlash against Moynihan's original 1965 report on the black family was the notion that it was unenlightened to suppose that people were handicapped by extramarital sex, divorce or spouse abandonment. Liberals were busily liberating people from antiquated divorce, abortion and sex laws. Those fights have been won -- or abandoned. If you had listened to Democratic congressmen or experts discussing poor families at their recent Greenbrier retreat, you would have heard celebrations of traditional values, an implicit concession that two-parent families and sexual restraint are better than the alternatives. This is obviously an issue on which right and left can come together. The right has to accept some government spending; the left has to honor traditional morality. The groundwork has been laid. A few years ago Rep. Barney Frank (D-Mass.) conceded that abortion opponents care about the quality of a child's life -- from conception to birth. Moved, Rep. Henry Hyde (R-Ill.) supported prenatal and infant nutrition programs, and suggested that right-to-lifers "have a positive obligation to protect a baby's life and health beyond the action of simply prohibiting abortion."

When the issue is posed that way, who can disagree? Those who do not share Frank's and Hyde's altruistic concern for children must pay heed to Moynihan's words: "the future of a society may be forecast by how it cares for its young." An age cohort that comes disproportionately from poor, disorganized, uneducated households will have a hard time producing as prosperous an economy and civil a society as most of today's adults would like to grow old in. A society of rich adults and poor children can become a society of poor adults, poor children and poor old people.

So it's almost certain that the issue of poor children will reach a high position on the nation's agenda, if not as legislation this year, then in 1987 and 1988. Ironically, the last two Reagan years could end up producing policies sharply out of line with the president's own preferences.