In fresh uniforms and school bandanas, Cuban children lined up on the airport tarmac to greet a visiting dignitary with songs and folk dances.

"The debt of Latin America and the Third World must be canceled," they chanted in clear, young voices directed by an earnest-looking teacher.

The sight of schoolchildren cheering for drastic international financial change, perhaps incongruous elsewhere, was just another sign here in Cuba of President Fidel Castro's increasingly intensive campaign to take the lead in the search for solutions to Latin America's dangerous foreign debt crisis.

In interviews, speeches and actions by his government, the Cuban leader in recent months has made the $360 billion Latin American debt burden his top public concern, eclipsing for the moment the Sandinista revolution and guerrilla war in El Salvador that absorb official Washington.

Much of that debt is owed by Argentina, Brazil and Mexico and not by Cuba. But Castro's adroit involvement in the issue is causing potential foreign policy challenges for the United States.

Repeatedly Castro has underlined the gravity of the crisis, calling the debts and interest charges impossible to pay. As a solution, he has suggested a sort of debtors' cartel and urged the U.S. government to assume the debts so U.S. creditor banks can release Latin American governments from the obligation to pay.

Castro, who for years has been regarded as the shaggy-bearded symbol of revolution in Latin America, also has accompanied his concern with warnings that failure to relieve the debt burden soon could lead to social upheaval across the continent. But rather than rejoicing in the prospect, he has stressed a need to avoid it, assuming the role of self-styled statesman eager to preserve Latin American institutions, including links to the U.S. banks holding most of the debt.

"Right now, there is something more important than social change," Castro told the Mexican newspaper Excelsior this spring, "and that is our countries' independence. This situation has brought Third World countries to such a state of dependence, exploitation, extortion and abuse that independence and struggle for the new international economic order have become the main issues for Latin American and other underdeveloped countries. Social changes alone are not the solution."

According to foreign diplomats stationed here, the roles of elder statesman and political sage have grown increasingly attractive for Castro, helping explain his exhortations about the need to "avoid explosions." The Cuban leader, whose beard is turning gray as he approaches his 59th birthday next month, relishes his place among Latin America's most durable and experienced leaders, they pointed out.

Expressions of concern about Latin debts also have fit in smoothly with the Cuban leader's increasingly successful effort to end his long isolation from other Latin American governments. Several formerly hostile governments have warmed to the revolutionary president recently and, diplomats here said, the United States has been unable to prevent the trend.

For these governments, Cuban chants about the debt crisis have been sweet background music for the rapprochement.

This is particularly so because Castro's theme has played to increasing fears in Latin America that Reagan administration emphasis on Central America's ideological confrontation has dimmed the view from Washington on what many Latins see as the more important economic crisis.

Castro's position has contributed to a sense of Latin solidarity, a Third World diplomat here pointed out, much as did Cuban backing of Argentina during the Falkland Islands war in 1982.

Despite opposition to the right-wing military junta then ruling Argentina, Cuba was careful to support Buenos Aires against the outsider British in that conflict.

"Fidel has struck another chord that began with the Malvinas," or Falklands, the diplomat explained. "And that chord is that we are Latin Americans, and we are countries in our own right, and we shouldn't let the United States push us around."

Talk of the debt crisis also gives Castro's government an opening to plug its relations with the Soviet Union as an alternative to most Latin countries' ties to the United States and U.S. banks, diplomats here pointed out.

U.S. officials have estimated that the Soviet Union provides Cuba with $4 billion a year in subsidized trade and cash. In a recent five-year economic cooperation accord, the Soviets suspended repayment of a Cuban debt to Moscow and Eastern Europe estimated at $9 billion.

"We have been able to postpone payment of our debt for 10, 15 and even 20 years without interest," Castro boasted in his March 21 Excelsior interview.

The official Cuban Communist Party newspaper, Granma, also emphasized this point for its readers in an editorial last week.

"Cuba is fighting a battle for the foreign debt problem and establishment of a new international economic order even though it is the least affected among Third World countries because of its relations of collaboration, exemplary and far from all selfishness, with the Soviet Union and the rest of the socialist community," it said.

Cuba also has trade relations with the hard-currency West, although they constitute only about 13 percent of the total. And despite his proclamations that Latin America's foreign debt is beyond payment, Castro's government has renegotiated its own $3 billion debt to Western European governments and banks.

Cuba's debt to western banks and governments, however, does not approach Brazil's $98 billion, Mexico's $96 billion or Argentina's $48 billion, Cuban officials have emphasized.

In this light, Castro has suggested that Cuba would remain outside any Latin debtors' club if such a group were formed as he has advocated. With or without Cuba, diplomats here pointed out, such a cartel likely would become a thorn in the side of the U.S. government since U.S. banks hold most of the debt.

Whatever the outcome, the Cuban campaign has been accelerating since it began in earnest early this year through speeches and a round of interviews.

The official trade union movement has organized a labor leaders' conference in Havana this month to discuss the debt. A debt conference of distinguished Latin Americans also is being organized for this summer.

Several Cuban officials interviewed on other subjects last week raised the debt issue spontaneously with a visiting American reporter.

"It is strange," said Education Minister Jose Ramon Fernandez, who is also vice president of the Council of Ministers. "The United States hardly ever talks about the debt, and they are right in the center of it.