It is always a wonder when the leviathan of government turns itself around. It has seldom been more gratifying than in the case of Amerasian children, by whom, at last, we are about to do the decent thing.

Last November, before a House committee, representatives of the Departments of State and Justice and the Immigration and Naturalization Service, like their predecessors in previous administrations, stood foursquare against any relief for children of American servicemen and Asian mothers.

The fear of fraud was prevalent. Much gibberish about a fancy new blood test, still in the experimental stages, was spoken. It was to protect us, supposedly, from being overrun by crafty mites who really had no claim on the privilege of living in the United States, since they might well be the children of Australian or German fathers.

But yesterday, some of the same people went before Sen. Alan K. Simpson (R-Wyo.), author of the omnibus immigration bill, and spoke of compassion and the need for prompt action to give these children, who may number 80,000, preferential immigration treatment, providing Americans will offer to adopt the younger ones and sponsor the older ones.

How did it happen?

Rep. Stewart B. McKinney (R-Conn.), who has been trying to get a break for the children since 1979, credited "citizen supporters who have developed a national lobbying network that would make many politicians jealous."

McKinney came to appreciate the problem of children that the U.S. government has preferred to ignore through Maryknoll priest Alfred V. Kean, a patient and persistent man who runs an orphange for Amerasian children in Seoul.

In the happy hubbub that followed yesterday's hearing, Kean looked like the center of the network. A U.S. Army colonel whipped out a picture of his adopted daughter, 7-year-old Keri. She was one of "Father Kean's kids, and we got her when she was 2 1/2," the colonel said. At Fr. Kean's behest, the colonel had written his congressmen and senators who stood ready to do whatever else was needed to bring other children here.

A woman whose husband had served in Korea and who had "fostered" several Korean children was another of those mobilized for the cause. Several young Amerasians, who may have to return to their native countries if Congress does not act, were on hand.

Rep. Barney Frank (D-Mass.), who last November crucified resistant administration witnesses, says the administration conversion was one of numbers. On May 19, the immigration, refugees and international law subcommittee of the House Judiciary Committee, voted unanimously to afford Amerasian children preferential treatment. Of the 180 sponsors of favorable legislation, more than half were Republicans.

In the Senate, the unexpected sponsorship of Sen. Jeremiah Denton (R-Ala.), the longtime Vietnam prisoner of war, gave matters a push.

All worked together to pass a bill that will put us in a class with the French for humanity. When they withdrew from Vietnam, the French took 25,000 children of mixed blood with them and provided for those left behind with their Vietnamese mothers. They offered French citizenship to all at the age of 18.

Under U.S. law, citizenship cannot be granted to children of male U.S. citizens if the children are born out of wedlock outside the continental United States, and the federal government has been free to pretend that the children did not exist. They are, of course, outcasts in Asian societies that pride themselves on "one blood" and where a child's whole identity and rights derive from the father.

The children's situation has been particularly pathetic in Vietnam, where they are regarded as "the dust of life" and treated with the contempt accorded human reminders of the long U.S. presence.

But there has been something of a turnaround there, too, according to a Vietnam veteran just back from a visit to that country. Tom Bird of Vietnam Veterans of America reports that Vietnamese Foreign Minister Nguyen Bo Thach announced a new policy of letting Amerasian children leave.

The Vietnamese had refused to acknowledge that children of U.S. servicemen and Asian mothers were abandoned or despised. Now they are apparently willing to allow them to emigrate to the United States "if the extraordinary departure process is a simple one" and if their American fathers offer to take them. That is complicated, but it is a beginning. The Vietnamese are ready to deal, even in the absence of diplomatic relations.

Now that our government has been educated by decent Americans, the stage is set to reverse a policy that, Fr. Kean pointed out, has led to the charge that "we are a country of barbarians who would abandon even its flesh and blood."

Some differences have to be worked out. For older children, adoption is not a real likelihood. But sponsorship for five years seems attainable, particularly when you see the dedication of those who worked to turn the government around.