THE FASTEST-GROWING sapling in the federal budget in the past few years has been none of the programs that you are accustomed to hearing the president and Congress debate. It is a system of federal payments to the states to help defray the cost of foster care. The government has been making such payments for 30 years but never in a big way. As recently as five years ago, the payments came to only $794 million -- yes, a lot of money, but in federal terms not enough to make it out of the fine print. Now suddenly the payments are $2.4 billion a year and in five more years will be $3.6 billion if cut, the budget office estimates, and more than $4 billion if not. A five-fold increase into the billions in 10 years is more than adequate to put a program up on the marquee.

What happened? Partly there has been an increase in the size and toughness of the problem, and partly what you see is an accounting change and an adventure in federalism. The states (though not all of them; see below) sensed an opening of the federal purse and quickly formed their line. The number of children in foster care fell for a while from its high of more than half a million in the mid-1970s, but now has started back up again. The data are notoriously poor for such a grave social indicator, but estimates are that the foster care population has increased by more than a third since the mid-1980s and is now approaching 400,000. That is only the average monthly case load. The number of waifs who, in the course of childhood, will have experienced a spell of foster care is much higher. Drug use and increased reporting of child abuse on the part of parents are among the reasons cited, but in some respects those too are only symptoms. More children are being born into tenuous families and non-families; that is the unblinkable fact.

As to who pays, in the last year of the Carter administration Congress passed legislation meant to discourage the warehousing of children in foster care. Among the provisions was an offer to help pay administrative costs, which were not clearly defined, in addition to the traditional maintenance payments to foster parents. States took the term administrative costs to mean, among other things, time spent by social workers determining whether a child belonged in foster care and getting the placement approved in court. That meant the federal government would start paying parts of state social worker salaries. The Office and Management and Budget through both the Reagan and Bush administrations has resisted in various ways, but so far the states have basically prevailed in both the courts and Congress. Nor in fairness are the states just laying off costs on the feds; their own expenditures have been rising too.

The federal government has never had much of an explicit family or children's policy, an omission which some people have deplored and others have thought a good thing. But partly in response to a disintegration of the family, that is changing. The government now has a special feeding program for pregnant women and infants. Fully a third of the children in the country under one year old are fed this way; the program costs more than $2 billion a year. The welfare reform bill of three years ago created a larger federal role in forcing absent fathers to pay child support. Last year Congress also authorized new federal subsidies for child care while also increasing a federal wage supplement for working families with children and providing that, over time, all poor children be afforded Medicaid. A kind of implicit federal children's policy is emerging on an anvil of need.