Sometime this fall, bouncy, blue-eyed Jessica Spiegel, age 4, is expected to sit on her mother's lap in a New Jersey courtroom and testify to alleged sexual abuse by her father when she was 2 1/2.

Legal experts say this apparently will be the first time a child so young is allowed to testify in such circumstances about events so far in the past. Word of the case, the result of a novel court decision in the wake of a messy divorce, has put a growing number of fathers' rights groups around the country in an uproar and added to a bitter national debate over child-abuse prosecutions.

Jessica first was asked to testify when she was 3 1/2. She spent most of the time crying or waving at her father across the courtroom.

Examinations by three doctors have failed to uncover physical evidence of abuse, Jessica's father said, but this is not necessary for a molestation arrest in New Jersey. So the father, Lawrence Spiegel, 39, stands accused of aggravated sexual assault, carrying a maximum penalty of 20 years in prison.

For Spiegel, a clinical psychologist, this Father's Day will be spent demonstrating in front of the home of New Jersey Gov. Thomas H. Kean, demanding a new look at molestation laws, and pondering the 18 months he has been denied contact with his only child.

Valori Mulvey, 26, Spiegel's ex-wife and Jessica's mother, could not be reached for comment. A woman who answered her telephone and identified herself as Mulvey's mother said she had been advised to say nothing until after the trial. Mulvey's attorney did not return a call.

Morris County assistant prosecutor Michael Rubbinaccio said he is barred from commenting on pending cases but that he carefully considers the age of a child witness and the complications of a bitter divorce before proceeding with a molestation case.

Spiegel's clinical practice is a shambles, although the same state agency that brought the charges recently let him escort a ward of the court, a 3 1/2 girl, to her relatives in Detroit. Unopened Christmas presents for Jessica sit in her empty room at his home in Flanders, N.J. Spiegel said during a telephone interview that his morning depressions are sometimes so great "I can hardly find the strength to pull myself from the bed."

"Much has been said and written lately about the possible trauma to a child who may have been sexually abused, if they are asked to testify," he said. "Yet nothing has been said about the trauma of a child who very possibly has not been sexually abused, who is not only made to testify against a parent, but who also must suffer the absence of that parent with no explanation."

Although no statistics are available, attorneys familiar with child-abuse cases say that molestation charges are becoming much more frequent in child-custody disputes. Fathers usually are the targets, but new civil rights groups concerned with the trend, such as Victims of Child Abuse Laws (VOCAL), have many female members.

VOCAL, founded following a celebrated Minnesota child-abuse case, has 52 chapters around the country and is leading the New Jersey effort to publicize Spiegel's story. It has called for laws that would allow prosecution of persons who make false child-abuse reports.

According to Spiegel, "my work, my dreams, my life and everything I had struggled for were shattered" the afternoon of Dec. 9, 1983 when county investigators arrested and handcuffed him in the parking lot of his Randolph, N.J., office.

He married Mulvey in March 1981, when she was pregnant with Jessica, a year after they met in a class he taught at a local college. The child was born April 6. His first marriage had ended in divorce after 13 years with no children.

After a year, his new marriage was in trouble. He and his wife sought counseling. But in January 1983 she left, unannounced, with their baby. When he tracked her down after three months and presented a court order giving him the right to see the child, she refused to honor it.

Under their October 1983 divorce agreement, he provided her with an apartment, nursery school fees and tuition to complete her college degree -- total support of about $16,000 a year. He retained rights to see Jessica two days a week and every other weekend.

Two weeks after the divorce, Mulvey took Jessica to Cincinnati. Court action by Spiegel brought her back, only to charge that she was trying to save her child from being sexually abused by him. With no evidence to support the accusation, the judge ignored Mulvey and gave Spiegel joint custody of Jessica.

Spiegel's attorney warned him to document carefully any time he might spend alone with his daughter. During one visit, Spiegel said, Jessica cheerfully told him for no apparent reason, "Daddy, don't put your face in my pagina sic ." Spiegel said he believed her mother had taught her to say that.

When Mulvey complained to the state youth and family services division about an alleged molestation one weekend in December 1983, Spiegel thought he was well prepared. A babysitter, his fiance and his first wife, who remained a good friend, had been with him and Jessica all weekend. Father and daughter had never been alone, he said.

Nevertheless, Jessica apparently answered affirmatively when asked by investigators if her father had touched her near her genitals. Spiegel was arrested and released on bail. The case went to court.

Her first time on the stand, Jessica simply cried. The prosecutors asked if the mother could sit with her, and an appellate court agreed to allow it just to see if her answers indicated she could qualify as a witness. At first, she simply responded to questions by waving, smiling and calling to Spiegel, "Daddy, Daddy."

After a recess, she said "yes" when asked if her father had touched her, and indicated the area of her buttocks. She also indicated that her memory was part of a "dream" and said "no" when asked if her father had hurt her.

The prosecution asked that her testimony continue on her mother's lap. Spiegel's attorney called this "akin to having Charlie McCarthy testify on Edgar Bergen's lap." Asked what would happen if she didn't tell the truth, Jessica testified, "My mommy will slap me."

But the appellate division of the superior court voted, 2 to 1, to allow the testimony. Two weeks ago, the state Supreme Court refused to review the case.

University of Nebraska psychologist Gary Melton, an expert on child-abuse law, said that some judges in the growing number of abuse cases are beginning to allow parents on the stand with child witnesses. But he said he had never heard of a child as young as Jessica being asked to recall events nearly two years in the past.

He said studies indicate that "recognition" memory, the ability to identify faces in a line-up, is well developed by age 4 but "free recall," the ability to recite past events, is far less certain, particularly if it concerns events on which the child was not focusing at the time.

Jessica qualified as a child witness when the court ruled that she met New Jersey's three-point standard: knowing the difference between truth and lies, showing a commitment to telling the truth and understanding that there is a penalty for not telling the truth.

Spiegel said he remains confident a judge eventually will dismiss the charges. He said the threat of jail bothers him far less than the time he has missed with Jessica.

"Unless the laws are revamped," he said, "any family in the country is vulnerable to the government coming in and taking their children."

Last Christmas Spiegel asked Superior Court Judge Charles Egan to let him see Jessica once during the holidays. Egan said that it was out of his jurisdiction because a similar request was before the appellate court. But Egan commented to the prosecutors opposing the visit:

"Let's assume, just for the sake of discussion, that you have an evil, venal woman who has put up her child to make scurrilous, unfounded, totally untrue charges against her husband or ex-husband, and because of the law's delay he becomes, in effect, a stranger to his child.

"I've always heard that in the law that there's a remedy for every wrong. But what's the remedy to this kind of separation? If Mr. Spiegel is totally innocent and is more the victim than the criminal? Doesn't that bother you just a little?"