IN an age of growing concern about the plight of American youth, San Francisco is considering a landmark children's initiative that could turn into a national crusade.Voters in the city - statistically the most adult in the nation - will take up a ballot measure next Tuesday that for the first time would guarantee a percentage of local spending for children's services. If approved, and polls suggest it will be, the measure is expected to be replicated elsewhere, as child advocacy groups across the country look for ways to garner funding for a group that has no voice at the ballot box. "The San Francisco initiative is a wake-up call," says David Liederman, director of the Child Welfare League of America, which is not involved with the measure. "It is being launched out of absolute frustration that we haven't been able to get support for what we believe is a top priority for the United States." Advocacy groups here have pressed hard for greater funding for children's services for the past four years. Upset at the response, they launched Proposition J. The measure wouldn't raise taxes to pay for programs. Instead, it would amend the city charter to mandate use of a portion of property-tax money for specific children's programs, such as child care, tutoring, delinquency prevention, job training for teenagers, and health and social services. "We mounted as sophisticated a lobbying effort on the local level for children as has ever been done," says Margaret Brodkin, head of Coleman Advocates for Children and Youth, a local group that wrote the initiative. "Our failure highlights how completely powerless kids are. We have to invent a new idea about what good government is when it comes to children." The drive here comes as concern grows nationally about the state of American youth. In a report in February, the Center for the Study of Social Policy, in Washington, D.C., declared the 1980s to be a "decade of deterioration for children." It reported dramatic increases in child poverty, births to teenagers, and violent deaths among adolescents. The National Commission on Children estimates 1 in 5 children lives in poverty. "We are clearly not investing enough in children," says Richard Lamm, former governor of Colorado and director of the Center for Public Policy and Contemporary Issues at the University of Denver. "This is more than a social tragedy. It is a nation-threatening trend." Mr. Lamm ticks off statistics that show what he believes are skewed priorities: the elderly receive three times more state, local, and federal assistance than do children, even though the elderly have the "largest amount of discretionary income in America"; the US ranks 19th in the world in infant mortality but has among the highest life expectancy of people over 80. Coleman Advocates cites its own chilling numbers to describe the decline of children in San Francisco: a child poverty rate above the national average; a 400 percent increase in child-abuse reports over the 1980s; the highest foster care rate in the state; juvenile-crime statistics more than double the national average. In addition to guaranteeing the current level of financing for children's programs - about $75 million a year - Proposition J would earmark 1.25 percent of property taxes for youth services in the first year and 2.5 percent in each of the next nine years. That would amount to $6 million in additional funding in 1992-93 and $13 million the year after that. No other city has done what San Francisco is considering. Others have tried tax increases to pay for children's services, with mixed success. Ballot efforts were turned down in Washington State and Fremont, Calif. One passed in Florida. The children's amendment, as the initiative here is called, is supported by a wide range of groups: from teacher and police unions to elderly groups to gay-rights organizations. Critics contend it will handcuff government, force cuts in other services, and take away the ability of elected officials to respond to needs as they arise. What would stop other advocacy groups from seeking funding guarantees? Arguing against the measure in the official ballot handbook, state Sen. Quentin Kopp (I) of San Francisco calls it a "seductive but deleterious concept." The Chamber of Commerce and local chapter of the National Women's Political Caucus oppose the measure. Seven of 11 county supervisors support it. So do three of five mayoral candidates. The city's two biggest newspapers are divided on the issue. Supporters have held a couple of rallies with children - including one at City Hall when 67,000 signatures were delivered in red wagons. They have released an emotional videotape of children describing, in Dickensian detail, tales of modern urban life. Although an August poll showed 75 percent of the people favored the initiative, no vote in California on fiscal matters is automatic. On Tuesday, children's groups here - and across the country - will be watching to see what San Francisco's grownups do.