THE family may be the most sacred rhetorical subject in the United States. The public attitude is always three cheers for the family. But when the time comes to convert cheers into cash, the crowd often melts away.Turning applauding onlookers into a paying audience is the daunting task facing the National Commission on Children in the wake of its comprehensive report on the needs of the nation's youth. Aptly titled "Beyond Rhetoric," the 500-page report, two years in the making, calls for more than $50 billion a year in tax relief and other programs to help families with children. With a unanimity rare on bipartisan commissions, all 34 members recommended giving parents a $1,000 tax credit for every child through age 18. Low-income parents who do not pay income tax would receive a check from the government. The panel also calls for tougher child-support enforcement, expanding Head Start to serve every eligible low-income preschooler, and access to good health care and education. Not all the commission's recommendations carry a price tag. The report also emphasizes the importance of moral values, urging adults to become role models for children and adolescents. Although the tax credit is a promising idea that deserves serious consideration, its $40 billion cost will require hard thinking about the size of the credit and how it will be paid for. At the same time, it would be unfortunate if the report becomes so closely identified with the tax-credit that budget-conscious legislators and taxpayers ignore its other recommendations. Critics are calling the report "a wish list," predicting it will be "a one-day story" because of the cost. But Democratic Sen. John D. Rockefeller IV of West Virginia, head of the panel, is vowing that "this is not going to be a commission that goes onto a scrap heap, I guarantee." His determination is well-directed. The report comes at a time when one American child in five is poor and one in four lives in a single-parent home. In addition, half a million babies are born to teenagers each year, and tens of thousands are born to drug-abusing parents. As the commission noted, "It is a tragic irony that the most prosperous nation on earth is failing so many of its children." The White House has acknowledged that the report contains "many good things." But it quickly added a stern cautionary note: "The fact is the money's not there." Yet billions of dollars do get appropriated for bailouts and weapons systems. Everybody agrees they hate war, but has any war gone unfunded? Everybody agrees they love the family, but has any family program ever been adequately funded? The commission report, as no report before it, calls the bluff of all those who say that nothing - repeat, nothing - comes before the family. Can Americans delay any longer the moral imperative to pay more than lip service to the next generation?