The youngsters who attended the congressional family performance of "A Christmas Carol" at Ford's Theatre yesterday and saw Scrooge in his nighshirt and Victorian top hat didn't recognize actor John Cullum at the party afterward.

Cullum, who plays the old tightwad transformed into a jolly-good fellow in the Dickens classic, came to the party in a white turtleneck with a dangling turquoise pendant. He allowed a younger member of the cast to pose in Dickensian attire for photographs with the children.

Wearing a "Bah, Hambug" tie was Clyde H. Wheeler Jr., vice president for governmental relations of the Sun Co., which sponsored the after-theater Christmas party in the tea room at Woodward & Lothrop.

Among the more than 600 guests at the performance yesterday was Amy Carter, who brought along two young friends and her Secret Service escorts.

David Pursley, who plays Marley's Ghost and Topper, the bachelor, among many roles in "A Christmas Carol," had changed from his Victorian yellow britches for the party. Pursley who lives in Greenwich Village two blocks from Ronald Reagan's recently married son, Ron, said the Secret Service had been canvassing the neighborhood for an apartment to rent.

"They were appalled at the rents. But no one wanted to give up an apartment," Pursley said, indicating that a lease in the neighborhood might unbalance the federal budget.

Members of the cast have learned to make quick changes during the Ford's production in what Pursley described as "a great flurry of scarves" behind the stage. One of them is 12-year-old Phillip Lewis of Chevy Chase, who in the last scene plays Turkeybone, a lad amazed to encounter the transformation of Scrooge. Having Amy Carter in the audience didn't make his performance any different yesterday, Phillip said.

"I couldn't see her in the audience when I was on the stage," he said.

Some 50 congressmen were there with their wives and children. Mayor Marion Barry and his wife attended the performance but did not bring their son, 6-month-old Marion Christopher. Among the congressional families attending were those of Sen. Paul Sarbanes (D-Md.), Sen Richard Schweiker (R-Pa.), Sen Joseph Biden (D-Del.), Sen Jim Sasser (D-Tenn.), Rep. Joseph Fisher (D-Va.), Rep. Jack Brooks (D-Tex.), and Rep. Thomas Ashley (D-Ohio).

For the children at the party, the entertainment continued after the theater show. Strolling guitarists wandered around the room, and magician David Willis performed a bag of tricks. A huge 19.5-square-foot gingerbread house dominated a buffet of cake, candies, cookies and candy canes.