THE LINE is already forming to spend the money the next administration will not have. Children's advocates are at the head of it. There are two great dependent groups in this as any society -- children and the elderly. The elderly, who vote, have been well attended to. Children have not. Their poverty rate is 20 percent (as against only 12 percent for the elderly and about 14 percent for the population as a whole). The infant mortality rate, after years of decline, has leveled off and in some states turned up again.

The Children's Defense Fund, the leading advocacy group, wants to require states to give prenatal care to all low-income pregnant women under Medicaid and health care to all young children in low-income families. Only some states do so now; the states and the federal government split the cost of Medicaid. CDF would also expand the WIC program that supplements the diets of these groups. CDF and others are pressing Congress, in addition, to begin giving federal day-care subsidies to lower income women and to increase federal funding of compensatory education programs. Normally conservative business groups have joined in calling for a broad children's program of this kind. The businessmen particularly fear for the quality of the future work force if no such steps are taken.

There is little opposition to these various proposals on the merits. The resistance has to do mainly with cost. CDF's agenda might, if fully funded, take $10 billion a year. The advocates say the country can't afford not to take the steps they propose, some of which they argue will have the net effect of reducing spending -- preventive health care costs much less than the alternative. Some of the advocates are also disposed to shrug the cost problem off as someone else's worry, much as Ronald Reagan did with the cost of the defense buildup. (Indeed, some would like to recapture for domestic programs some of the money he shifted to defense.) But unfortunately the country can't afford to be so blithe about costs again because Mr. Reagan has already spent a lot of his successor's money; the next administration and the children's groups will have to choose among their goals.

The health proposals are the most compelling. America has slipped to 19th in the world in infant deaths. More black children in large U.S. cities die before their first birthday than babies born in Jamaica or Costa Rica. Black babies are twice as likely to die as white babies, but white infant mortality has also risen lately in 19 states. The number of low-birth-weight babies in the United States is increasing for the first time since 1960. Such babies are more likely to die or be impaired. Prenatal and postnatal care has been shown elsewhere to make a difference. Medicaid is already in place and,in fact, has been quietly extended in the past several years in precisely the direction the children's advocates want to go. The WIC program is also established. These programs can be affordably expanded. So perhaps can federal aid to education, though this remains essentially a state and local responsibility.

The day-care proposal is a different matter. Conservatives as well as liberals have lined up behind day-care bills in Congress, which loves to enact brave new programs in election years. But here the government would be taking on a major new obligation when it can't fulfill the heavy fiscal and social obligations it already has. Day care is a serious problem, but so are a lot of things. Congress shouldn't promise what it can't afford.