The Reagan administration said yesterday that, in ordering home the allegedly abused 9-year-old son of a Zimbabwean diplomat, it was trying to uphold the principles of diplomatic immunity and assure the boy's welfare once back in Zimbabwe.

White House spokesman Marlin Fitzwater said President Reagan was "satisfied" that Zimbabwe had strong enough child-protection laws, at least "on the books," and had been assured by Zimbabwean authorities that the boy, Terrence Karamba, would be given "full protection under the law."

In a statement on the unusual case, State Department spokesman Charles E. Redman said the United States had held "extensive conversations" with the Zimbabwean government and been told that the boy will be placed in the custody of a local social welfare society and not returned to his family.

The boy "would be entitled to protection of the courts through a court-appointed guardian," Redman said. "No one is talking about turning him over to his father."

The State Department official said that the U.S. government has also had contacts with independent legal authorities and groups dealing with child welfare in Zimbabwe and that "private organizations that are active in the {child welfare} field in Zimbabwe have all indicated to us that they will closely monitor his situation."

The case has pitted the State and Justice departments against New York City welfare officials and the Legal Aid Society over the fate of the boy, who was removed from his family Dec. 11 and placed in foster care after the city charged that his father, Floyd Karamba, had abused him.

The State Department has expelled the father, an administrative attache with the Zimbabwean mission to the United Nations in New York.

Both the State and Justice departments, worried about the legal implications for U.S. diplomatic families abroad, have asked that the boy be returned to Zimbabwe. New York City officials want him turned over to a foster family and granted asylum in the United States.

The 2nd U.S. Circuit Court of Appeals is scheduled to hear the case today.

In a statement, Redman said the department felt it had not made clear that its "central concern" had been the child's welfare.

But he said this had to be reconciled with international law regarding diplomatic immunity, which prevented the State Department from prosecuting the boy's father.

For the same reason, the department has been arguing that neither New York State authorities nor the federal courts have jurisdiction over the boy, he added.

"This is a principle that has important practical aspects for the United States as well. We have thousands of U.S. government personnel overseas, and diplomatic immunity is important to protect them against illegitimate exercises of jurisdiction by foreign governments that might seek to intimidate them or their families," Redman said.

But he added that international law also requires that the host government protect the safety and welfare of persons with diplomatic status. "That's precisely what we have done and continue to do with respect to Terrence Karamba since he asked us to protect him from his father," he said.

Redman said the department had not sought the transfer of the boy to the custody of the Zimbabwean mission in New York until it had expelled his father, "thus removing the immediate threat to his safety."