It's an elegant party in an elegant home, with people smiling and drinking punch and dancing to music by Tchaikovsky. Since it's Christmas Eve, guests have brought along their children. And children, even in 19th- century Germany, often misbehave. One boy, arriving with his family, makes a snowball and pelts it at a guest.

The music stops and we're hurled into the 20th century, into a mirrored studio at the Washington School of Ballet, where Martin Buckner, wearing a ski sweater and taking notes on a yellow legal pad, is directing a rehearsal of "The Nutcracker."

"You see the snow," he says, scooping up an imaginary handful. "You've been waiting all year for this . . . heh, heh, heh. . ."

It's four days to opening night and Buckner is slightly harried, trying to get the 72 children in the opening party scene to do every bit of business just right. Buckner has brought this on himself: When he and Mary Day choreographed the Washington Ballet's version of the traditional Christmas ballet, they emphasized the children's roles.

"I got the inspiration from Dickens' Pickwick Papers, where he talks about the party scene in 'The Nutcracker,' " says Buckner, clicking on the music to start the party again. "Directing children would be fine if I had just one cast, but there are two casts of boys, four casts of girls, two casts of parents, and one cast of directors!"

The next time the music stops, it's owing to cast confusion, which has reduced one young dancer to tears.

"I just missed my group," he explains.

"This is the first cast, is it not?" says Buckner. "You're in the third cast."

An adult dancer comforts the child, and the party goes on. Someone passes out fruit -- real -- and the guests dance with it. Then Fritz, who in the ballet is the son of the hosts but in real life is 10-year-old Syd Butler, dances with a hobby horse he takes from under the Christmas tree. This is his favorite part in the show, says Syd, a blond boy in a Lacoste shirt and jeans. He likes it even better than the part where he taunts his older sister, Clara, and breaks her doll.

"Do that whole thing again," Buckner says to Clara, played today by 13-year-old Kristina Windom, who is dancing around with the doll. "You're doing nothing but looking in the mirror."

Kristina explains that she's looking in the mirror to ascertain the whereabouts of her brother, but Buckner tells her to concentrate on the nutcracker doll.

"If Fritz is out of sight, that's great," he says, then adds to Syd, "You're not bratty here. You just want to get hold of that nutcracker in the worst way."

They dance again, and Buckner approves: "There . . . that's lovely . . . good."

The hall outside the studio is crowded with dancers dressed in leg-warmers stretching their muscles, and with parents -- many of whom spend as much time at the studio as their children.

'I don't even remember if I like it -- it's just what I do," says Kay Butler, mother of Syd and 13-year-old Katherine, a former Clara who is a Candy Cane and a doll in this year's Nutcracker. Katherine takes classes at the school every weekday and Kay, a board member, comes every day, too, to help out and plan fundraising events. Syd, she explains, is not really a ballet dancer but has been in the show for three seasons now.

"They asked the girls if they had any brothers," explains Butler, "and Syd had no trouble playing a precocious younger brother. I think it's wonderful for him to be able to understand what his sister does."

Robbie Wallace, 17, who is changing from boots to ballet shoes to rehearse his roles as the Nutcracker prince in one cast, a mechanical doll in another cast and a shepherd in still another cast, switched from soccer to ballet four years ago.

"I had been active in sports and when I saw Baryshnikov dance, I thought I'd like to try that. Then I just took off. . ."

"Way off -- up into the air," chimes in Mary Barton, explaining that the great heights Robbie reaches are much admired by the other dancers. Barton, a member of the Washington Ballet's apprentice company of talented teenagers, plays shepherdess to Robbie's shepherd and also dances with the flower corps, which is waiting for a rehearsal with teacher Robert Steele, ballet master of the apprentice company, who's busy rehearsing the snowflakes.

"Temps lev,e, balanc,e, temps lev,e, balanc,e," Steele chants as a score of graceful snowflakes swirl and sweep across another studio. There's one cast of snowflakes and two casts of flowers, and their dancing must be coordinated with that of the Snow Queen and the Sugar Plum Fairy and their Cavaliers, played by members of the Washington Ballet company.

"They're on tour in Singapore, but they're due back tomorrow," explains Pam Pauker, registrar of the school whose clipboard is a permanent appendage. Attached to the clipboard are lists of the casts, which Pauker has organized to accommodate siblings and aid carpools.

"Find jabots!" says a reminder on the bulletin board in the costume room in the basement, and Philip Toro, 8, is trying on the rest of his costume, sans jabot.

"Do the pants fit, Philip?" asks Caroline de Wineberg, a parent and costume mistress. "My son plays the role in another cast. Is he the same size? He has to be!"

People who play the same roles in different casts are supposed to fit into the same costumes, but since the the roles are sometimes played by different dancers each year, there are always alterations to be made. There are racks and racks of costumes -- furry teddy bears and mouse heads, silky Chinese outfits, elegant party dresses. Then there are lists of which costumes are complete and which ones need work.

"I'm always happy when a red X finally appears after a costume on the list," says Pat Bloom, another parent-costume mistress. "There's one mother who takes three costumes home every time she brings her children for classes. She makes sure all the hooks and eyes are right and does the hems. This show is not only kids, but parents who give their time, too."

Kristina has come in for a final fitting on the nightgown she wears to fall asleep and dream on the Kingdom of Sweets, and she and Philip talk about Christmas. He wants an Atari game, but for Kristina there are no visions of sugarplums.

"I'd like a foot massager," she says. NUTCRACKERS, NUTCRACKERS WASHINGTON BALLET -- "The Nutcracker" opens this Friday night at 7 at Lisner Auditorium and plays through December 31. Call 362-4644.

This Sunday, the Washington Ballet will offer a benefit party at which children and parents will eat holiday sweets and chat informally with the costumed Nutcracker cast. A $25 ticket includes a 5 o'clock performance and a party at the Arts Club of Washington. For $50, toe shoes worn and autographed by the Sugar Plum Fairy are included. Call 362-4644. VIRGINIA BALLET COMPANY -- Presents "The Nutcracker" at Woodson High School in Fairfax, December 26 through 30. Call 321-8009 or 830-3267. ANNAPOLIS CIVIC BALLET COMPANY -- Will dance "The Nutcracker" this Saturday at 2:30 and 7:30 and this Sunday at 3:30 at Annapolis Senior High School. For information call 301/263-4755. MARYLAND REGIONAL BALLET -- Presents its version this Saturday at 2 and 8 and this Sunday at 2 at The Weinberg Center for the Performing Arts in Frederick. Call 301/694-8585. PROCHOTSKY BALLET THEATER -- Will dance "The Nutcraker" this Saturday at 2 and 7 and this Sunday at 1 and 5 at Walt Whitman High School in Bethesda. Call 962-4908 or 948-1078. BOB BROWN PUPPETS -- Presents a puppet version of the story at the Capital Children's Museum Fridays at 11:30, Saturdays at 11 and 2 and Sundays at 2 through January 2. Call 543-8600. I