Three small children sat on the floor in a Georgetown office suite one recent Saturday intently practicing on a doll how to diaper and dress a baby--skills they would later teach to 20 expectant parents.

Both the children and the parents were enrolled in sibling classes at the Birth Education Center, 2262 Hall Place NW. The center, a year-old nonprofit operation established by registered nurse Zoila Ortego Acevedo, is the first of its kind in the District.

In addition to the sibling classes, Acevedo teaches prenatal and postnatal courses, Lamaze preparation, Caesarean preparation, a pre-pregnancy course and parenting and adoptive parent lessons.

Her 10-hour sibling course, taught in five Saturday morning sessions for a $20 fee, prepares parents and their children for the often-difficult adjustment of young children to the arrival of the baby. In addition to the infant-care practice, the children, usually ages 4 to 9, study the birth process and the parents learn breathing and birthing techniques.

Acevedo, 35, is one of 7,500 certified birth-education instructors in the country and "has outstanding qualifications," according to Robert Moran, executive director of the American Society for Psychoprophylaxis and Obstetrics in Rosslyn. He said Acevedo, who has a doctorate in human behavior, passed his organization's two-year certification program.

Moran said he knows of no other classes in which parents and children learn together as at Acevedo's center. "I think it's a marvelous idea," he said.

"It is unfortunate that before the birth some parents prepare for the coming child without giving the present child the same opportunity," said Acevedo, a Cuban native raised in the United States.

"Existing children should be incorporated into classes so birth is not a sudden happening but is digested, prepared for and awaited. The earlier children prepare for this new life, the easier it is for them to come to grips with it."

Acevedo said preparation "can't help but cut down on the sibling rivalry and negative feelings that most children have toward a prospective brother or sister."

In addition, she said, preparation helps balance "unrealistic expectations" that the baby will be like a doll or a playmate, who is "warm, cuddly and quiet. In truth, most babies cry a lot, wet a lot and have to be fed a lot and aren't at all like dolls."

Joy O'Toole, mother of 4-year-old Brooke, said that before her family attended the sibling classes, "Brooke burst into tears and said my stomach would split open and the baby would fly all over the house, creating a bloody mess."

The course helped to allay Brooke's fears, however, her mother said, and "birth looked like an easy thing to her. She knew where the baby came from . . . . It was more enriching for all of us to have taken the classes."

At one recent class, Eric Siegel, 8, turned in his homework: a drawing of himself and of the baby girl his parents expect next month.

"Look, the baby has one hair," Acevedo exclaimed. "You drew the placenta and the umbilical cord too. Nice picture, Eric," she praised the beaming boy.

Richard Siegel, budget director for the City Council, said he and his wife, Laurie, brought Eric to the class because "we want him to know the experience from conception to birth. . . . He would develop anxiety and jealousy if we hadn't brought him to the classes."

The group was shown illustrations of how a fetus develops. The parents then went to a room to pratice breathing techniques. Meanwhile, the children, with the help of Acevedo's mother, Yara Ortego, prepared for "show off time."

"It's hardly ever that kids know how to do something and grownups don't," Eric Siegel said.

"I'm not scared. They're grownups, not Martians," retorted Acevedo's son, Eric, 8, who had been enlisted by his mother to help make the others feel at ease in the class.

Maura Finkelstein, 3, sidled up to economist Rick Stuckey, 27, saying reassuringly, "I'll show you." When Stuckey lifted the practice doll by its leg and not very gently, she groaned along with the rest of the class.

Eric Acevedo used a real baby, Maura's 3-month-old brother, Michael, to demonstrate diapering. "Don't go all over me," he ordered the baby, who had just finished nursing. "I'm on your side."

Afterward, about half the parents said they learned something useful from the children's demonstration. "When putting the sleeper on, put the feet in first," recited Richard Siegel.

"Be very gentle and cover up a baby boy so he won't wet you," chimed in expectant father John Baker, a Metrobus driver.

"Zoila's a first-rate professional," said Donna Glenn, who with her husband, Mike, and son, Jason, 9, attended Acevedo's classes before their daughter, Rory, was born last year. "She's been on the firing line as a nurse and has the additional theoretical and practical training for childbirth."