Jack Child, a retired lieutenant colonel who put in 22 years in the U.S. Army, two tours in Vietnam, and three years teaching at West Point, will pull on his green combat fatigues today and report to the Vietnam Veterans Memorial. For the first time in his life, he will march in an antiwar demonstration.

Simultaneously, two blocks away from the V-shaped memorial, Milt Copulos--also a veteran of two tours in Vietnam, who was evacuated with injuries and saw many friends die in combat--will speak at a rally for the first time in his life, to a crowd directly opposed to Child's viewpoint.

The holiday weekend demonstrations, focusing on the Reagan administration's growing involvement in El Salvador, reflect the increasing national concern about the violence and tension in Central America. They also evoke the bitter and seemingly endless divisions caused by the Vietnam War and by the black granite memorial itself.

The opposing rallies mark the first mass demonstrations to be held near the memorial that lists the names of the 57,939 men and women who died in Vietnam. Organizers of both groups say the crowds will be peaceful, although the expected presence of some radical elements on both sides has prompted the National Park Service to plan for a sizable police contingent at the two demonstration sites, near 21st and 23rd streets and Constitution Avenue NW. Police are expecting a combined crowd of 5,000 to 6,000.

Child, 45, now an associate professor of Latin American studies and Spanish at American University, will join several thousand people expected at the demonstration organized by the "Ad Hoc Committee for July 2 Emergency Mobilization," a broad coalition of veterans, liberals, leftists and labor organizations who see in El Salvador the makings of "another Vietnam."

Copulos, 35, now director of energy studies at the Heritage Foundation, will speak at a rally of the "Captive Nations Vigil Committee," a recently formed group of veterans, conservatives and supporters of U.S. intervention in Central America who believe the American role there is crucial to stopping the advance of communism.

For Child, who still speaks in the clipped cadences of a career military man, the occasion marks what he calls a "coming out" in publicly condemning "misguided" American policies that he believes may force many of his students and his two sons, Andrew, 18, and Eric, 15, to fight and perhaps die in an unpopular and futile war.

"I have the gut feeling that we have been down this road before, and that each of us needs to decide soon whether we will speak out and act in time, or live with our silence later, as I did in Vietnam," Child said. "I have recently made my personal decision, and it is to come out of my closet and speak out."

Child, who voted for Barry Goldwater in 1964 and later for Richard Nixon at a time when the country seethed with antiwar sentiment, has worked for the Joint Chiefs of Staff, has been an intelligence officer in Saigon, and has a nine-page, single-spaced resume attesting to his expertise in military affairs and Latin American studies.

Military and academic colleagues describe him as an able officer and a gifted teacher and researcher, with generally centrist political views. "Jack is a reputable Army officer, and he was outstanding in the academic area," said his former boss, retired Lt. Gen. Ranald T. Adams. "He is also a very opinionated and strong-minded guy."

Child said that when he saw the posters for the antiwar rally as he was walking through Adams-Morgan a few weeks ago, he didn't sleep well for several days as he did some soul-searching about whether to join. He said he fears that by participating, he could lose many of his military friends and possibly jeopardize his chances of getting tenure at AU.

But he said he feels so strongly that the United States is headed for danger that he not only decided to march, but also to publicize his action. "I have a somewhat unique perspective, as a career officer . . . and I want to make a statement. I don't want to be just another face in the crowd," he said.

Child, who went to Vietnam in 1961 as part of the first big wave of military advisers under Gen. Maxwell Taylor, said he supported the war initially but that the U.S. got "sucked into" taking over a losing battle for a corrupt South Vietnamese regime, instead of merely offering assistance.

Child said he has been deeply frightened by the unfolding scenario of America's step-by-step involvement in El Salvador, which he said parallels the Vietnam experience: first, economic aid; then military aid; then advisers and later combat-training specialists; then a few American casualties; then medical support units; then combat units to guard the support troops, and slowly, but surely, full-scale combat.

He said he believes America's security must be defended in Central America, but said the only way to stabilize El Salvador is through an international peacekeeping force, along with a multinational effort to create sound democratic institutions, reduce economic and political injustices, and curb militarism and right-wing violence.

The rally Child will join has been strongly condemned by the Veterans of Foreign Wars and the Vietnam Veterans of America--with the VFW calling it "a mass obscenity" that desecrates the new memorial and dishonors the war dead. VFW national commander James R. Currieo blamed "far-left activists" and "aging anti-Vietnam War activists" for using the memorial as a political platform.

But Child, who describes himself as "a patriotic and loyal American, in the best sense of the word," said he saw the demonstration as a tribute. The best way to honor the 57,939 dead, he said, was to assure that America "must never again get sucked into a situation like Vietnam without a clear purpose, a cause we can believe in and strong public support for our armed forces."

On the other side, Copulos said that when he heard of the Ad Hoc Committee's rally at the memorial, he was overwhelmed with "intense anger . . . that these people were trading on the names of the men who died in combat."

He was particularly disturbed, he said, because the Ad Hoc Committee includes "an awful lot of people very sympathetic to the Marxist revolutionaries in Latin America and to the regime in Hanoi."

Once the July 2 rally was announced, Copulos helped organize the Captive Nations counterdemonstration "because this is something that is so important I felt it was necessary to have someone speak out who was a veteran" and still supported the cause in both Vietnam and in El Salvador.

The Captive Nations rally, he said, is meant as a "dignified" tribute to veterans, living and dead, "who performed honorable service for a just and proper cause."

He added, "the justice of the cause is underscored when you look at . . . these literally billions of people in the world who are suffering under totalitarian regimes, like the one imposed on the Vietnamese people.

"I don't want to see the U.S. have another Vietnam either," he said. "I don't ever want to see us commit ourselves to a conflict where we are not really committed" to an achievable goal. Copulos disagrees with the suggested Vietnam-El Salvador parallel, because, he said, El Salvador's proximity makes it vital to U.S. interests and the Salvadoran government has more popular support than media reports suggest.

The Ad Hoc Committee, which claims the support of more than 500 groups in nearly 100 cities, said its march from the memorial to Lafayette Square will be led by a contingent of veterans and their families, including a group of "Gold Star mothers" whose sons won combat medals and were killed in action.

The Captive Nations group, formed only three weeks ago, was attempting this week to line up added support and recruit speakers. The group held a small 24-hour vigil starting at noon yesterday at the memorial, followed by today's rally two blocks from the memorial.