Washington — More than 3 million children have fallen into poverty since 1979 - the sharpest increase in poor kids ever recorded. That, at least, is the conclusion of the Children's Defense Fund, a nonprofit lobbying group. The CDF says President Reagan has slashed $10 billion from federal programs that help children, and the group is urging Congress to reverse the cuts.
''Poor babies are being born daily and they deserve the best chance to live that we can give them now,'' insists Marian Wright Edelman, president of the fund.
The effect of Reagan's budget cuts on the poor is the subject of much debate. In a study widely quoted by Democrats, for instance, the Census Bureau estimates that 15 percent of the US population is now officially poor, the highest level since 1965.
David Stockman, director of the Office of Management and Budget, says in reply that this figure ignores the growth of government aid programs such as food stamps, housing and health programs, etc.
How the poor have fared since Reagan took office is, in essence, the heart of the so-called ''fairness'' issue. Against this background, the Children's Defense Fund's just-released study on child poverty is sure to be contentious.
As a result of budget cuts during a recession, the study says, 700,000 children have been thrown off Aid to Families with Dependent Children, and medicaid. Four hundred forty thousand have lost education aid. Thousands of mothers, says the report, have been thrown off maternal and child health services.
And if past history is any indication, the CDFclaims, even a sharp economic recovery will pull only about one-third of the newly poor children out of their economic straits.
In particular, the CDF is worried about these trends, according to Marian Edelman:
* A nationwide decrease in the percentage of pregnant women receiving prenatal health care.
* Glimmers of an increase in infant mortality. Though progress continues in many areas, in 11 states the infant mortality rate increased between 1981 and ' 82. It now appears as though the US will not meet the surgeon general's 1990 goal of 9 deaths per 1,000 live births, says CDF.
* The falling percentage of poor children enrolled in medicaid. In late 1976, 83.5 percent of children living in poverty received medicaid; that figure has since fallen to 73.5 percent.
* The falling percentage of poor kids helped by Aid to Families with Dependent Children. In 1979, 72 percent of poor children were on AFDC. In 1982, the figure was 53 percent.
* Dwindling federal education help for disadvantaged children. Poor children, the CDFsays, are disproportionately hurt by such actions as Reagan's '81 withholding of $440 million in primary and secondary school funds.
And putting money into children could save the government money in the long term, claims CDF. ''It pays to invest in kids,'' Edelman says.
The CDF cites a '78 Harvard study, for instance, which estimates that each dollar spent on prenatal care can save $3 in postnatal hospitalization costs.
And it's much cheaper to keep a poor family together by giving them AFDC, says the Children's Defense Fund, than to support the children in foster homes or institutions.