Kevin Harris stood on the sidewalk in front of the White House yesterday, his shrill, 10-year-old voice rising above the shuffle and chatter of tourists and passersby.

"Dear President Reagan," Kevin said, reading from one of a stack of letters that he held in his small hands. "Please don't spend the money that my parents worked so hard for to blow up the world."

The Children's Campaign for Nuclear Disarmament had come to Washington, bringing with it 40 youngsters, mostly preteen-agers, bearing what they said were 2,802 letters from youths across the country asking President Reagan to stop the nuclear arms race.

As the youngsters read the letters aloud in front of the White House, Nessa Rabin, the 12-year-old organizer of the event, waited anxiously for the highlight of the event: an appearance by Reagan to accept the letters.

To her disappointment, Reagan did not show up.

Most of the mail that was read was critical of the Reagan administration's arms policy.

"You older people have already lived most of your life, and us younger people would like to have a chance to live ours," one letter read. "No matter what country started a nuclear war, our whole world could go out like a light bulb, and there would be no one to turn it on," said another.

Rabin, who lives in Plainfield, Vt., said she and some friends got the idea for a children's campaign soon after the last presidential election.

"I became very concerned when Reagan went into office that the arms race was out of control, in my mind," she said.

Nuclear disarmament was also the topic on the other side of the White House yesterday at the Washington Monument grounds where the Caravan for Peace and Human Survival held a rally.

The rally was one of five separate caravans heading to the United Nations in New York for a rally on Oct. 24, which is U.N. Day. The caravans are being sponsored by the Campaign for U.N. Reform, the World Federalists Association and a group called Planetary Citizens.