It was a hot summer day when Sharon's 6-year-old daughter, a pretty, small-boned girl, went to visit a next-door neighbor in her public housing project in Southeast Washington.The neighbor was not at home, but her 18-year-old son, who often baby-sat the girl, was.

The youth invited the child into the house. Moments later, he undressed her and then raped her.

Sharon's daughter was one of thousands of children who are sexually assaulted each year in the nation by relatives, friends or strangers.

No one knows exactly how often child sexual abuse occurs, but the National Center on Child Abuse and Neglect estimates a minimum of 100,000 cases a year.

"It is without a doubt the most underreported of child-abuse cases," says Kee McFarlane, a specialist in sexual-abuse problems for the National Center on Child Abuse and Neglect.

Perhaps more telling than the figures are the deep psychological scars often left on sexually abused children. Reactions vary from withdrawal and extreme anxiety to fear of the dark, fear of strangers and regressive behavior such as nail-biting and bed-wetting. The child's performance in school can also be adversely affected, with truancy, shortened attention span and poor grades often the result.

In the case of Sharon's daughter, "it was pitiful," says Joyce Thomas, head of the child protection unit at Children's Hospital where the girl was brought after the assualt. "She's a very small child. She was curled up in the fetal position and sucking her thumb."

Physicians at the hospital examined the girl and determined that she had been raped.

"The mother was outraged," Thomas said.

McFarlane says such cases of sexual child abuse are doubling each year, though she is not sure whether the reason for the growth is that there are more incidents or that there is simply more public awareness and willingness to report those incidents. Latest figures show 6,000 reported cases of sexual child abuse involving family members in 1978, twice the 3,000 reported in 1977. cThose figures do not include the thousands of cases in which children are sexually abused by friends or strangers -- as distinct from family members -- and reported to police, McFarlane said.

"Sexaul abuse has been around since the begining of time," says Thomas at Children's Hospital. From January 1978, when the child protection unit was established there, to November of this year, 850 cases of child sexual abuse were reported. The victims ranged in age from 3 months to 17 years.

Officials say the child victims of sexual abuse cross all economic levels, from the poor in the ghettos to the rich in affluent suburbs. At Children's Hospital here, officials said cases involve mostly poor families from the inner city, while in other places like Harbor View Medical Center in Seattle, officials there said their patients are mostly middle-class family members.

Between 85 percent and 90 percent of the victims are sexually abused by someone they know, officials say. The majority of the persons committing the child sexual abuse are men and the victims girls, McFarlane said.

McFarlane said the average age of known victims of child sexual abuse is 6 to 9 years, much lower than the average of 13 to 14 years a decade ago.

It was July 1, 1978, when Sharon's 6-year-old daughter was raped by the 18-year-old neighbor in Southeast Washington.

"I couldn't believe that one of my neighbors that I really trusted could do this," said Sharon (not her real name), touching her short black hair as she sat at her dining room table. "What really hurt the most was that someone I knew and trusted would do something like this to my little girl. Someone who had baby-sat for me. Someone who I had shown around my home.Someone who I thought I knew so well."

Sharon found out in a roundabout way that her daughter had been raped. An older brother of the neighbor had walked into the house and discovered the 18-year-old sexually assaulting the little girl. He told the girl's brother in a conversation overheard by the girl's 5-year-old sister, who in turn told the mother when she came home.

To her dismay, Sharon learned after talking with her 6-year-old that the incident was not the first. The same neighbor had raped her daughter on a previous occasion.

"I wanted to tear down the [neighbor's] house," Sharon said. She reported the incident to the D.C. police, who took the child to Children's Hospital. The neighborhood youth was arrested and charged with rape.

Afterward, mother and daughter underwent extensive therapy. "The mother had a great deal of anger she had to get out," said Thomas.

The child's mother said she also did not know how to react to her daughter. "I didn't want to let her out of my sight," she said. "I would keep her around the house. I wouldn't let her go out and play with the other kids. . . I guess I was getting next to her. She was getting next to me. It went on for a year before I could really deal with it."

Even today, Sharon said she still gets angry when she thinks about what happened. The youth, who pleaded guilty to a lesser charge of performing indecent acts on a minor and was placed on three years' probation, still lives in the neighborhood. Sharon and her family have tried to avoid him, but that is nearly impossible. "If I see him leaving out of his house when I am outside, I will go back in the house," she said.

Sharon, who works as a supervisor for a downtown hotel cleaning staff, said she wanted to start a petition to prevent the youth from living in the project, but neighbors did not want to get involved. She wanted to move her family but couldn't afford to. One neighbor, she said, told her that if her daughter "weren't so hot," the rape would have never occured. "She acted like my daughter was some full-grown 16-year-old. She was 6 when it happened. She wasn't nothing but a baby. She doesn't have anything to impress a 18-year-old man."

She said she thinks her daughter has adjusted since the incident, although she really is not sure. "She doesn't talk about it," Sharon says. "I'm always wondering what effect this will have on her. What kind of child will she be? What kind of woman she will turn out to be?"

Sharon, who also has four boys besides her two daughters, said she worries about all her children, but especially the girls. "I told them don't be asking nobody for nothing," she says. "Stay out of men's and boys' faces.

"You know, every time I heard about a girl being raped, I think about the mother. I really know what they are going through. . . .It's just a thing. It's something I will never forget."

Thomas said Sharon's reaction to her daughter's rape is typical. She said because the rape was committed by someone the mother trusted, the mother was deeply hurt. "You can always accept fate," she said. "When you are in a situation when it is someone you trust, it can be devastating."

Often with poor families, Thomas said, "Parents freak out. They are dealing with a crisis. Oftentimes, these parents have other problems, and this is like the straw that broke the camel's back."