Thank God for Gretchen. She's the 13-year-old who seems to understand President Reagan's Central America policy. At least, he apparently thinks she does, although he seems to be not entirely clear about it himself.

Gretchen is the latest in a line of young females who have served world statesmen by exhibiting wisdom obviously denied their elders.

She follows Amy Carter, whose father, Jimmy Carter, presented her in his debate with Reagan as a nuclear stateswoman although she was but 9 at the time, and Samantha Smith, Yuri V. Andropov's 11-year-old pen pal from Maine, who is just home from two weeks as a good-will ambassador to the Soviet Union.

The president quoted approvingly from a letter written by Gretchen, who says she is pretty exasperated with people who can't grasp what he is saying about--or at least relate it to what he is doing--in the Latin American world.

"Don't you wish sometimes you could just stamp your feet and shout at the press or senators to be quiet, and sit down and listen to what you're saying?" Gretchen asked him in a letter that he conscientiously admitted he has yet to answer.

Actually, people have been listening a little too closely of late, and the president had to have a news conference to tell them not to be scared that he's starting a war just because it looks as though he is.

He seemed rather scared himself at his news conference. He sighed and hesitated a lot, and used "maybe" and "perhaps" more than usual. Last Thursday, calling up the fleet, announcing the dispatch of thousands of GIs to Honduras and turning aside queries about a massive spook buildup, he had sounded like Teddy Roosevelt--"Remember the Maine!"

On Tuesday, however, in the East Room, he seemed to be trying to pass for Mohandas Gandhi.

He was like a driver who has been caught doing 80 in a 20-mph zone and keeps telling the cop that there has been some dreadful mistake--"I had the brake to the floor, officer."

Make war? Get us into another Vietnam? How can people be so suspicious? A man of peace like him? His performance recalled the old joke about the man who, when accused of pretentiousness, replies, in a wounded tone, "Moi?"

The president went out of his way to be nice to the press, to suggest that it is all one big happy family over there.

He made a point of recognizing three rookies to the press corps. He got nothing for his pains.

All three asked about Central America. The first, a young woman, raised the same picky argument. Wasn't there, she asked "something contradictory" about a show of force to show how much you hate it?

"Well," the president began, and answered the question by asking another one. "Wouldn't we be sending them some kind of a signal that might be the wrong kind of a signal to send if we want peace in that area?"

The commander-in-chief doesn't know about everything we are doing to calm things down in Central America, in spite of the nosy Latin neighbors in the Contadora Group, who can't seem to grasp the idea of peace with gunboats.

There's some confusion in his mind about the details. He isn't sure how long the warships are going to stay in the Caribbean.

He's vague, also, about the number of GIs he is sending to Honduras--it's "between" 3,000 and 4,000. As for the spook increase, "No one has presented a proposal to me."

His eye is not on the sparrow, it's on the dove.

He was, by contrast, terribly explicit about a Soviet freighter which is steaming toward the Port of Corinto in Nicaragua. He knows its name, the Ul'yanov. He knows what is on it, too--"military equipment, helicopters, transport helicopters for military purposes and so forth . . . ."

Reagan may be sending weapons there, but not for military purposes.

"We want to underscore once and for all that the United States . . . seriously opposes the use of force by one neighbor against another," he said in his opening statement, " . . . and to the export of revolution."

That was as clear a statement of principle as Bianca Jagger could ask.

How does it apply to the exported, violent revolution we are sponsoring against Nicaragua? Would that not be, in CIA terminology, a candidate for "termination with extreme prejudice."

Many in Congress wish to vote it down, but that, it seems, would be wrong. Reagan said, "It would be a "very grave mistake if the legislature interfered with what we are trying to do"--which, everyone had plainly heard him say, just minutes before, was "to see an end to violence and bloodshed."

Maybe the peasants being shot by the "contras" we pay will not mind so much if they know that Reagan's motives are peaceful.

Gretchen, apparently, can follow it all without any difficulty. What in the world is wrong with the rest of us?