The U.S. Embassy's youngest hostage had an emotional reunion with his mother today when the militant Moslem captors holding him and 49 other Americans agreed for the first time to let a relative of a captive into the occupied compound.

Barbara Timm said after a 45-minute meeting with her son, Marine Sgt. Kevin Hermening, 20, that she would go back to the United States to work for a "peaceful settlement" of the hostage crisis by pushing for congressional hearings on past U.S. involvement in Iran.

Officials in Washington said they planned no legal action against the Timms, although President Carter had barred Americans who are not journalists from traveling there. White House press secretary Jody Powell and State Department spokesman Hodding Carter III noted that the ban officially goes into effect Wednesday. The Timms "got in under the wire," Carter said.

[President Carter, on CBS, said of Timm, "My heart goes out to her. I have no intention of punishing her."]

[When asked whether possible military moves against Iran might endanger the lives of the hostages, Carter responded: "I consider them in jeopardy now. There is a volatile political situation in Iran . . . I constantly worry about and think about and pray about the saftey of the hostages. I don't know how much longer we can sit here and see them kept captive while the situation (deteriorates)."]

Appearing composed and self-confident at a news conference after returning from the embassy tonight, Timm said she found her son "in excellent health and very, very happy to see me. He was surprised and overjoyed that I had traveled across the globe to be with him."

She said that their meeting was filmed throughout by an Iranian television crew as several of the militants stood by. She said that the captors did not tell her whether or when she would be able to see her son again, and that there was no indication whether other relatives of hostages would be allowed to make similar visits.

Timm said she did not appeal to the captors for Hermening's release or discuss with them any prospective move toward that end.

She said she agreed before the meeting not to discuss anything political with her son, an arrangement that she said was "just fine with me."

Most of the 45 minutes she and her son spent together was taken up by news of the family and their hometown, she said.

The militants initially agreed to give Timm a half-hour with her son, she said, but eventually allowed them an extra 15 minutes.

"We never quit holding hands," Timm said. "There was a lot of hugging, a lot of touching. There were no tears. I kept telling him how strong he was, and he kept telling me how strong I was. He says he has not lost faith. He said, I've became a better person and a stronger person.' I told him I had come to give him strength and faith."

Timm, of Oak Creek, Wis., was brought into the embassy through a rear gate at about 3:15 p.m. After three of the Moslem captors took her and her husband, Kenneth, to see the graves of Iranians killed during the February 1979 revolution that deposed shah Mohammad Reza Pahlavi.

One of the militants, known to U.S. television viewers by the pseudonym "Mary," told the couple at the Behesht-e-Zahra cemetery south of Tehran, "We are friends of the American people, but the American people must ask their government to return this criminal [the shah] to Iran so he can be tried."

Timm said later that she thought the militants made the decision to allow the visit while driving back to Tehran from the cemetery.

"I knew that they had to test me," she said. "They wanted to feel me out. They didn't want to be bombarded with someone who was going to go in there and rush them."

A half hour after the Timms arrived at the embassy, Mrs. Timm and Hermening, her son by a previous marriage, were brought together in a room at the rear of the embassy compound while Mr. Timm was led to another room. Hermening was not informed that his mother would be visiting until 20 minutes before the meeting.

The Timms were in the embassy about six hours in all, Mrs. Timm said, and most of the time was spent with the militants.

"We talked and we talked and we talked," she said of her meeting with the militants. "I still can't understand or justify an embassy being taken over. But I had to live with one of two choices; either going on hating these people and having that destroy our family, or trying to understand these people."

She said the captors showed her "nothing but the best of hospitality," adding, "I found human beings."

Timms said her son, who was posted to the embassy as a Marine guard two months before it was seized, has gained two pounds and spends most of his time "with several of the other hostages mostly reading and playing cards.

"The government has said these people are brainwashed," she said of the hostages. "I really don't know what that means, but I can't agree. What would be the sign?"

Some of Timm's remarks at the news conference were critical of the Carter administration. She, her husband and their lawyer, Carl McAfee of Norton, Va., denied that the militants were using their visit for propaganda purposes or that the captors had taken advantage of them.

Timm said, "I want to see a peaceful settlement. I don't want to see sanctions, blockades or military action. I don't want to see any innocent people die."

She said that upon her return home, she would work for the holding of congressional hearings on the U.S. Iranian crisis. Timm did not say, however, whether she had any indication from the militants that this would lead to the hostages' release.

An unusual statement of support for Timm and her efforts to obtain her son's release came from one of Iran's most powerful revolutionary judges, Ayatollah Sadehgh Khalkhali.

He urged that Hermening be released, saying "We are not enemies of the great American nation . . . but love them." Iran's real enemies, he said, are the "ringleaders of corruption, who are led by Carter, Kissinger, Goldwater, Nixon and Johnson."

Khalkhali, who has claimed responsibility for sending hundreds of Iranians to firing squads after the revolution, said that "to demonstrate our good will toward the American nation and the hostage's parents and because the hostage's parents have come all this way to meet their son, we must not only allow them to meet him, but I also seriously demand that the authorities free this hostage to demonstrate our interest in the American nation."

In the CBS interview, the president said that he preferred that other families of hostages not go to Tehran, but stopped short of prohibiting such visits. "I would have to make a judgement on the final action in each case.