Almost all the UNICEF donations made to children going door-to-door in the Washington area this Halloween will be earmarked for the aid of famine-plagued Cambodians, the executive director of the United States UNICEF committee said yesterday.

C. Lloyd Bailey, executive director of the committee, said donations from the metropolitan area would go to UNICEF's Cambodian relief fund unless donors specifically request otherwise. The UNICEF drive gets its funds in small change collected by Halloween trick-or-treaters who carry special donation boxes along with the bags they use to solicit treats for themselves.

The earmarking of Washington-area funds was prompted by Rep. Michael D. Barnes (D-Md.), who called a news conference on Capitol Hill to urge area residents to increase their gifts this year.

Flanked by entertainers Danny Kaye and Hugh Downs, both of whom work with UNICEF relief efforts, Barnes displayed a letter from Kelly Brantland, a young constituent, who said:

"I have read in Time magazine that people are starving in Cambodia. As my congressman, could you please help them?

"Please vote to send more money for food."

Taped to the letter, Barnes said, was a dime. With the notation: "My donation."

James P. Grant, director-general-designate of the United Nation's Children's Fund (UNICEF), said at a news conference that the Washington-area initiative "could set a pattern for the rest of this country.

Barnes noted that providing relief "cannot be done entirely by government. It takes people, too. And we, like no other nation, have a history and tradition of our people exceeding their government in compassion and commitment and contribution to relieve the suffering of people in need."

Last week, President Carter pledged approximately$69 million in relief aid for Cambodia, the Southeast Asian country torn by war and ravaged by famine.

The House voted last week to ensure there would be enough money to meet Carter's pledge. The measure still must be approved by the Senate.

Barnes said he is convening a meeting Nov. 6 of Montgomery County religious and civic leaders to organize a countywide fund-raising program for Cambodia relief.

Grant said recent logistical agreements made with the Vietnam-backed Cambodian government of Heng Samrin ensure "there is no question that the money made available to UNICEF will get in, and the things that it buys will be reasonably well used."

UNICEF has started a nationwide, $110 million fund-raising drive on behalf of Cambodia. Nine other national charitable organizations, including the American Friends Service Committee and International Red Cross, also have launched Cambodian relief drives.