"The children of Sandino don't surrender or sell out," goes a verse in the revolutionary song that now serves as Nicaragua's second national anthem. "We fight against the Yankee, enemy of humanity."

Since the Sandinistas came to power here 11 months ago, the Carter administration has been trying to change at least the second part of that tune.

There is still mistrust on both sides, but in recent weeks what some diplomats here call a maturing of the Sandinista leadership and breakthroughs in the U.S. Congress on important aid to this desperately poor nation have combined to improve the tone of relations between Nicaragua and the United States.

There is still resentment here for the support the United States gave to the widely hated Somoza family dictatorship for nearly 50 years. But influential Nicaraguan leaders such as junta member Sergio Ramirez now talk of defining "a new field of relationships" with the United States built on mutual respect.

House majority leader Jim Wright (D-Tex.) was sent here with three other congressmen by President Carter this weekend to reinforce the positive steps that have been taken. Arriving in a presidential jet emblazoned with the name of the United States, the emissaries found themselves warmly -- if sometimes curiously -- received.

They met with members of the government, leading Sandinistas, businessmen, journalists and diplomats. The visit was given front page coverage by all three of the newspapers in Managua, including the Sandinista newspaper Barricada.

When he left Tuesday morning, Wright suggested that "hiding under the scowl of anti-imperialism" -- which is often taken to mean anti-Americanism -- "there still is a residue of good will for the United States."

Many Nicaraguans would agree, but some political observers here think a more pragmatic factor is at work.

"There is more understanding here that Nicaragua is a small, poor country with a big, powerful neighbor and that that is simply a reality," said Alfonso Robelo, a former junta member who now heads an opposition party. "We will have a lot of relations with that neighbor, especially economic relations, and most of our problems are economic.

"During the first euphoria after the war we tended to ignore that," Robelo said, "But now that the euphoria has passed and they have visited Russia a few times, the Sandinistas have come to realize it."

Money is a key factor in the new relationship between the United States and Nicaragua. It has proved difficult for a Congress wary of communist influence to give and it is difficult for the Sandinistas government to accept.If strings are attached to the money, the Nicaraguans run the risk of appearing to have "sold out."

The halting progress which Congress of $75 million special supplementary item in the 1980 aid budget has been watched closely here. Conditions have been attached to it and occasionally it has seemed close to death. This has changed it from the symbol of friendship that the Carter administration intended into a symbol of American bad faith and interference in the minds of many Nicaraguans.

Last month's final authorization of the aid, which still must survive the complex appropriation process, was the first positive step in months from the Nicaraguan point of view. When the House last week defeated an amendment that would have killed almost half of the $54 million budgeted for Nicaragua in 1981, relations moved ahead again.

At the same time, the Nicaraguan government has resolved its worst internal poltical crisis since the revolution precipitated when Robelo and Violeta Chamorro resigned from the five-person junta, not by moving further to the left as some observers predicted, but toward the center.

The appointment of two more moderate, non-Sandinista members to replace them last month, reassured both Nicaraguan business interests and the United States.

U.S. anxieties have also been eased in recent weeks by agreements reached between the leftist Sandinistas and the nation's private businessmen. The agreements promise a review of state confiscations of land, implement a law allowing private citizenz to take the government to court, and the post-war state of emergency and promise the announcement of a date for elections.

A major concern on Capitol Hill, however, is that Nicaragua is already so inclined toward the East that it must eventually become a Soviet or Cuban satellite. The Carter administration and supporters of aid to Nicaragua are convinced that to withhold money will virtually guarantee such an outcome.

"Our visit shows we're not going to bail out and leave it to the Russians and Cubans," said Rep. Stephan L. Neal (D-N.C.).

But the congressmen were aslo somewhat surprised to find that the Soviets, with whom the Sandinistas have established cordial and extensive relations, have been encouraging the Nicaraguan government to take much the same path to socialism that the United States has indicated it finds acceptable.

The Soviet ambassador here recently told several political leaders that the Marxist-Leninist elements of the Sandinista front should be able to coexist with a prosperous private sector and maintain good relations with the United States.

Such remarks have led some observers to conclude that the Soviets want to let the United States foot the bill for a revolution that is already theirs. U.S. Ambassador Lawrence Pezzullo argues, however, that the Soviets simply "recognize that such plans are a reality here and it would be as silly for them to argue against such a reality as it would be for us to."

The Nicaraguans are strong nationalists and most observers here believe that it will be a balance between that nationalism and their leftist ideology that will keep them out of the Soviet camp. Both Soviet and U.S. diplomats recognize, however, that the Sandinista leadership is more responsive to suggestions when it is applauded rather than when it is attacked. r

"The money is only part of the need here," Rep. Bill Alexander (D-Ark.) said. "The greatest need is the moral support and the friendship of the United States."