ONE USEFUL side effect of the Reagan administration's assault on the domestic budget is that it is forcing Congress and the public to take a good look at where the federal dollar goes and to decide what kinds of purposes those dollars should support. One of those purposes got a resounding vote of confidence in the House yesterday, when a resolution reaffirming federal responsibility for child nutrition programs was passed virtually without opposition.

In the administration's New Federalism scheme, child nutrition programs are to be lumped together with other social service programs and "turned back" to state and local control. For an interim period, the federal government would also provide funds roughly equal to current costs, but in 1987 the programs would be repealed, federal support would begin to be phased out, and states would be left to decide whether or not to pay for the programs.

The likelihood that many states and localities either couldn't or wouldn't continue child nutrition programs has aroused numerous educational, social service, religious and agricultural groups. They argued before a House subcommittee last week that the failure of states to make up any of the $1.5 billion already cut from these programs shows clearly that states and localities can't be counted on to take care of the low-income children who receive almost all of the benefits from school lunch and other nutrition programs.

They were backed up by Sen. Robert Dole in an unusual surprise visit to the House subcommittee. Sen. Dole, who is cosponsoring a similar resolution with strong bipartisan support in the Senate, observed that federal programs have made enormous progress against child malnutrition over the last decade. A federal role is needed, he and others argued, because children are the society's most vulnerable group. Adequate food is a basic need, and that need may be greatest where the local resources are least.

Those are precisely the claims that should be weighed in deciding what national purposes the federal government will continue to support. State and local officials meet with President Reagan today to resume discussion of his federalism proposals. The House resolution reminds them that both Congress and the public have something to say about the appropriate division of their responsibilities.