An antique Santa Claus, his robes still cherry red, hangs at the very top of the 20-foot Veitchii fir in the center of the Blue Room of the White House.

Underneath Santa are 2,500 antique and near-antique toys from the Margaret Woodbury Strong Museum in Rochester. N.Y. While the museum staff was decorating the tree, H. J. Swinney, the director, said, "Amy came down to watch and we put her to work unpacking things. She had a good time arranging furniture and dolls in the Victorian doll house. It worked out well. She's a nice child. We would never have trusted our museum-quality pieces with an obstreperous one." Rosalyn Carter, showing the tree to the press yesterday, pointed out the Byelo doll, which she said was the best-seller of all time until the Barbie. "Amy likes the Barbie-type better than baby dolls," she said.Mrs. Carter herself played "Jingle Bells" on the toy piano to show how she felt about Christmas, but Amy, she said, can play "The First Noel" on the violin.

The other decorations this year were done by Dorothy Temple of Holston, Mass. Decorations are simpler this year-green rope, pine cones (some from Georgia) and cyclamens with tall undecorated cedar trees in the crosshall.

The creche, given to the White House by Jane Englehard and her husband Charles 11 years ago, has had somewhat of a population explosion-10 more figures added to the other Neapolitan 18th-century characters. The background has been redesigned by painter Loretta Howard of New York as a mountain-side so it fits better into its alcove in the East Room.

The tree in the Blue Room is only one of the Carters' trees this year. They have one upstairs in the family quarters of the White House. And just before Christmas they'll go home to Plains, fo out into the woods and chop down one for their own home. Mrs. Carter said their Plains tree will be decorated with ornaments made by retarded Georgians, given her when President Carter was governor.