The parents of the youngest American hostage arrived here today in hope of obtaining his release, but the Moslem militant captors holding him and 49 others said they were still discussing conditions under which the couple could be admitted to the occupied embassy.

Barbara Timm, accompanied by her husband, Kenneth, and a Virginia lawyer, said that as a first step she wanted to meet the militant Moslem students who have been holding her son by a previous marriage, Marine Sgt. Kevin Hermening, 20, for the last 5 1/2 months.

At times fighting back tears that threatened to shatter her composure, Mrs. Timm, 41, of Oak Creek, Wis., told a news conference here that she longed to see her son, but that she also wanted to meet Ayatollah Ruhollah Khomeini and Iranian President Abol Hassan Bani-Sadr and to understand the Iranian revolution.

"We want so deeply to meet with the ayatollah," she said. "We are prepared to get down on our knees and plead for an opportunity to meet with him. Certainly, we want very badly to see Kevin, but that is not the sole purpose of coming to Iran. We would like to learn to understand the people. We would like to understand their thinking, to have an opportunity to converse with them."

But the closest she came to meeting any Iranians holding the hostages on the first day of her visit here in definance of new U.S. sanctions against Iran was a drive by the eight-foot-high brick wall of the embassy compound. She paused briefly to gaze past rifle-toting Revolutionary Guards through the embassy's front gate without leaving her taxi.

Inside, the militants evidently were divided on whether and under what conditions to allow the first visit by a hostage family since the embassy takeover Nov. 4.

A spokesman for the militants said the Timms would be allowed to visit their son "under some conditions which will be announced later." The spokesman said the conditions would apply to all families wishing to visit their captive relatives.

Reminded that Bani-Sadr and Foreign Minister Sadegh Ghotbzadeh have said that families would be permitted to visit the hostages, the militants' spokesman said, "Under our conditions, that will be possible."

[State Department spokesman Hodding Carter said the United States will bar the families of other American hostages from traveling to Tehran because they could become hostages themselves. "It is just recognition of the fact that we are in sort of a quasi-war situation," Carter said.]

The Timms' lawyer, Carl McAfee, of Norton, Va., said he had been told that the White House has fouled up a "piecemeal release" at Christmas of five or six hostages, including Hermening. McAfee also took strong exception to a White House spokesman's comments that the family's visit to Iran would be used "cynically" by the militants.

"We do not look at the hostage situation the same way as the White House, and we see nothing wrong with piecemeal releases," McAfee said.

McAfee said that he was hired by the Timms because of his previous experience in such situations. He represented the families of U2 spy plane pilot Francis Gary Powers, a crewman of the U.S. Navy ship, Pueblo and prisoners of war and soldiers missing in Vietnam.

"Each one of those hostages does not stand as an equal to the others," McAfee said. He indicated that Hermening, 20, should receive special consideration.

When Mrs. Timm was asked whether she would seek her son's release, McAfee answered for her: "That certainly is the objective any parent would have. Barbara would like the opportunity to plead her case with someone. Someone must have ultimate authority."

Mrs. Timm described how her and her husband's feelings had changed from hatred of Iranians to sympathy for them. Several of her remarks were critical of the U.S. government's handling of the hostages crisis and she expressed strong opposition to any American military action.

She said she had brought her son several Christmas and Easter gifts and letters from Hermening's brothers and sisters as well as photographs of them.

"We want desperately to see Kevin," she said, "but we have a deeper reason for being here, and that is to understand the people of Iran. We want to understand what happened to make them take over an embassy. We want to understand why their feelings are so deep toward the shah. We would like to know, from them, what they have against the American people."

The Timms said they had first tried to come to Iran in January, but that the Iranian Embasssy in Washington refused their applications for visas, apparently in retaliation for the escape from Iran of six Americans at about the same time.

"When the takeover took place last November, we were first faced with a very deep fear that our son would die. When a week or so passed and they were not harmed physically, we then started becoming filled with hatred, and that hatred stayed within us for over a month, where we truly hated the Iranian militants."

Then, Mrs. Timm said, her feelings alternated between hatred for Iran and for the American government.

She said the turning point came earlier this month when she and her husband watched American television film of Iranian diplomats and military students being expelled from the United States.

"We felt the same deep infuriation that these injustices were being brought against these people," Mrs. Timm said. "At that moment we decided to come."

Mrs. Timm said that before the trip, she had seen her son in television film shot during Easter ceremonies. "He looked in good physical shape," she said. "But he looked very sad. He looked like he was very bewildered, that he honestly didn't understand why this was happening."

Mrs. Timm told the news conference, "We do not believe that anybody in this country is going to physically harm my son. I fear deeply for my son's life if our president carries out his threat of military [action]. I do not want to see any of those 50 hostages killed. I do not want to see one single Iranian killed as a result of the fact that there cannot be found a peaceful way to end this crisis."

Asked what she would say to her son if allowed to see him, Mrs. Timm said, "I've thought about that since the day of the takeover. I'm sure when I see him I'm going to turn into a babbling crying mother."