Last year, the children of Washington's diplomatic community got Mr. T as Santa Claus. Yesterday, they got a new Mister and a new Santa as Mister Rogers left his Neighborhood to visit the East Room of the White House -- bringing along much of his TV show for the annual Diplomatic Children's Christmas Party. And jolly old Willard Scott, who normally plays Santa on the "Today" show, donned a red costume and felt once again what it was like to have hair on his head, though this time the hair was white.

The halls were alive with the sound of music as the Marine Corps Orchestra performed seasonal favorites inside the White House entrance, but the symphonic harmony was undercut by the buzzing of many little tongues in many native dialects. The children wore colorful dashikis and Korean silks, Dutch and Indian outfits, suits and ties. They were large and small, like the countries they come from, with name tags suggesting an intimate geography of the spirit.

And while many of the children seemed genuinely pleased to see Mister Rogers and Mr. McFeely (the Neighborhood mailman), Betty Aberlin and those rather dour puppets, King Friday the 13th, Queen Sarah and Daniel Tiger, many also seemed puzzled. That may explain why the chorus for "Tree Tree Tree, We Love You" was not as enthusiastic as the cheer that erupted when Mrs. Reagan suggested an end to ceremony and a beginning to cookies.

King Friday and Queen Sarah exchanged Christmas greetings with President Reagan (who could not attend) and the first lady; and Mister Rogers positively beamed when he looked out on the mini-United Nations and said, "I'm so glad we could be real neighbors today."

Moving to the West Room for refreshments, the children managed to ingest every cookie in sight. They crowded around Nancy Reagan, who graciously dispensed signed White House cards as if they were McNuggets coupons, and around Mr. McFeely, no doubt trying to inspire speedy delivery of overseas packages. Asked whether they had preferred Mister Rogers or Mr. T, the children proved perfectly diplomatic. "I liked them both," said Asha Nortus, 11, of Belgium, her sentiment echoed by Marina Shcherbavkov, 10, of the Soviet Union.

Just before the East Room buzz, there was a special private Christmas ceremony for 5-year-old Amie Garrison of Clarksville, Ind. Amie, who was born without bile ducts, is in immediate need of a liver transplant. The White House learned that Amie had wished that her favorite ornament be placed on the White House Christmas tree. As a result, Amie and her family were flown in from Louisville, where she is under specialists' care at Norton's Children's Hospital, so that she could hang it herself. Sitting in a stroller and looking somewhat dazed by the flash and whir of cameras, Amie managed to attach her Santa to one of the tree's lower branches while Mrs. Reagan gently stroked her hair and whispered private words of encouragement.

As Amie left, she waved gently to Santa Willard.