In 1941, when war overshadowed Washington's festive Christmas season, Winston Churchill stood with Franklin Roosevelt at the lighting of the White House Christmas tree and said, "Let the children have their night of laughter."

Last night, the voice of Churchill resounded on tape through the old Pension Building as past and present Christmases in the capital were celebrated. Dozens of Washington children, along with President and Mrs. Reagan, participated in the taping of a television special, "Christmas in Washington." The show will be seen tonight on NBC at 10 p.m.

While a camera, decorated with an oversized red bow, captured the caroling of Diahann Carroll, Debby Boone, John Schneider, Dinah Shore and Ben Vereen, the Reagans clapped and sang along. "He's always taken with this kind of thing," said Attorney General William French Smith of the president's joviality.

The Reagans had two roles: In the first the president's voice was heard, on a tape made Friday at the White House, reading a Christmas story to some of the children of White House staff members, and Mrs. Reagan, also on tape, reviewed the history of Christmas in the White House. In their second role, the first family walked to the red-and-white stage and the president told how he had called the mayor of Bridgeport, Conn., yesterday, praising a community effort to rebuild a burned-out facility where the children annually receive Christmas presents. "They had 250 volunteers working around the clock," said Reagan. "Yes, Virginia, there is a Santa Claus."

The show, which was held at the building now called the National Building Museum, opened with the national anthem. George Stevens Jr., director of the American Film Institute, who planned the program, called the 800 people crowded on gold chairs in a portion of the building "dense pack," a reference to Reagan's proposed basing plan for the MX missile.

Last night's gathering was a Washington event that no one criticized. "I'm finally getting the Christmas spirit. Before I was only doing my budget," said Secretary of Health and Human Services Richard Schweiker. "Now I'm going to get Dave Stockman in the Christmas spirit." Grant Tinker, chairman and chief executive officer of NBC, and Stevens were receiving shouts of congratulations from all around the room. "This is a good, warm warmup for Christmas, a kickoff," said Tinker. After the taping, among those found swarming around the tables of oyster stew, ham and biscuits, gingerbread men and cranberries in wassail bowls were Interior Secretary James Watt, presidential assistant Michael Deaver, White House social secretary Muffie Brandon and newsmen Roger Mudd, of NBC, and Eric Sevareid, formerly of CBS, who both had parts in the program. Backing up the entertainers were the U.S. Naval Academy Glee Club and the Shiloh Baptist Church Choir of Washington. "They've been in rehearsal for 12 hours. Someone had seen their appearance last Easter on national television, and that's how we got the call," explained the Rev. Henry Gregory III, the pastor of Shiloh.

By the end of the evening, nearly 400 presents had piled up under the tree, including one from the Reagans and one from Vice President and Mrs. Bush, designated for the patients at the Children's Hospital National Medical Center. The hospital also received a $25,000 outright gift from Warner Communications Inc., which paid $75,000 for last night's party. While everyone was making plans for the holidays, John Schneider, who starred in CBS' "The Dukes of Hazzard" until a legal dispute with Warner Bros. last summer, said he would be in a recording studio in Los Angeles working on his fourth album. "But I'm smiling and, if all goes well, we'll start back on 'The Dukes of Hazzard' on Jan. 3."