Patti Linebaugh still remembers having to identify the body of her 2 1/2-year-old granddaughter, Amy Sue Seitz.

The little girl was kidnaped, tortured with a pair of pliers and raped in 1978. Sun and water had so distorted her features that Linebaugh at first could not make a positive identification. A year later, when they found the man who had done it, Linebaugh was stunned. Here was a lean, distinguished-looking student of sociology who had been a known child molester for 20 years.

Six weeks before Amy Sue's death, Atascadero state hospital had pronounced him cured and released him as a model of what could be done with modern psychiatry.

"I guess you had an angry grandmother then," Linebaugh said. "I wanted to make sure that no child would have to go through what Amy did."

From this hilly, rapidly growing suburban community northwest of Los Angeles, the impact of Patti Linebaugh's anger has spread so far it could well curtail the treatment of child molesters throughout the country, sending them instead to long terms in prison.

The citizens group Linebaugh founded, Society's League Against Molesters (SLAM), has already forced the addition of mandatory sentencing to California's molester law and eliminated automatic hospital treatment for many convicted offenders.

Through personal friendships, exchanges of letters and news clippings, SLAM and similar groups have also forced changes in the laws in Colorado and Washington state and taken their work into New York, Michigan, Texas, Ohio and other states. It is the beginning of a movement that may rival the success of the citizen groups that have succeeded in securing toughened laws against drunken drivers.

Everywhere in the country, legislators, columnists and angry citizens have begun to propose longer jail terms as a protection against individuals suffering from severe, difficult-to-alter social problems, putting them on a collision course with the rising costs of courts and prisons.

The American prison population increased 12.1 percent in 1981, the largest annual increase since figures began to be compiled in 1925. The total of 369,009 individuals in prisons breaks all U.S. records, and some lawyers say something has to give.

"What happens after they pass mandatory sentencing is that the prison poulations double or triple very quickly," said David Landau, legislative counsel for the American Civil Liberties Union. "They become breeding grounds of crime." The overcrowding then leads to judicial orders that can force early releases of some prisoners or massive expenditures of tax money for new facilities.

Families, police and prosecutors who have immersed themselves in unusually disturbing cases of child molestation seem more than willing to pay for more prisons. Irving Prager, the Ventura County deputy district attorney who prosecuted the Amy Sue Seitz case, now teaches law and is preparing an article on child molesters. "It is the most misunderstood crime in America," he said.

Prager said some studies indicate that a majority of child molesters were themselves molested when they were children. Some researchers speculate that molesters are creating a new generation of disturbed individuals. The crime is doubly difficult to identify and stop because so few victims are willing to tell their parents or the police what happened to them.

Courts in most states, including Maryland, Virginia and the District, rarely give lengthy jail sentences to child molesters. Instead they prefer to put molesters on probation on the condition that they undergo psychiatric treatment.

"To steal a TV set is considered more of a crime than child molestation," Linebaugh said. Prager said: "A true pedophile, whose true sexual preference is with prepubescent children, is a danger to children all his life, and at least should not be allowed around children."

Specialists in treating molesters differ over whether some offenders, such as relatives of the victim or young offenders guilty of only one incident, might be helped by therapy.

Patrick Barker, a psychologist who helped treat Theodore F. Frank, Amy Sue Seitz' convicted killer, at Atascadero, said, "If the person himself or herself truly wants to make a major change in his life," then therapy can work. Barker said it was "a horrible, horrible feeling" to discover that the patient released with his and others' approval had killed a little girl. He said he knew then that Frank "had wanted to persuade Atascadero that he wanted to make the change" when actually all he wanted was the early release from confinement.

When the California legislature decided last year to curtail its mentally disturbed sex offender program so that molesters could no longer be released on a doctor's approval before their prison term was up, therapists like Barker supported the change.

Other therapists, however, like Donald T. Lee, mental health clinical district chief in Los Angeles, opposed the change because it removed what they considered a useful incentive to persuade offenders to undergo treatment in the first place.

Lee said his program of carefully monitoring the activities of released offenders has reduced their rate of new offenses. "What people do not realize or want to think about is that sooner or later everybody comes out of prison," he said.

Diane Serber, chief of forensic services for the California department of mental health, said, "The data that has been gathered so far is just not very good. We do not have data that shows conclusively that the treatment has helped." But, she added, the data also does not prove that the treatment does not help in some cases.

Often educated and knowledgeable about what psychiatrists consider signs of improvement, chronic offenders have managed to turn the criminal justice system into a revolving door. SLAM's Colorado chapter began when Becky Davis of the Denver suburb of Littleton met Bea McPherson, who had been a neighbor of Linebaugh here. Both were outraged that a man previously convicted of sexually molesting an 11-year-old boy and beating him with a table leg had been given only a 90-day jail sentence for a second offense, sexually molesting a 9-year-old girl. With help from Gov. Richard D. Lamm, they secured passage of a mandatory four-to-eight-year jail sentence for second-time child molesters.

Washington state recently changed its laws to allow earlier prosecution of molesters. In New York, residents of the cities of Holtsville and Mastic Beach have formed groups to press for tougher state laws.

Linebaugh's daughter Sheryl had not married Amy Sue's father and the couple was separated when the little girl was killed. She has since married another man and now has 4-month-old twin boys, but her mother says she still tries to separate herself from any reminders of Amy Sue's death or similar tragedies. "She doesn't get a newspaper, she doesn't listen to any news," Linebaugh said.

The case of Theodore Frank, 47, provides a chilling footnote to the contradictory academic reports on the treatment of sex offenders. According to his probation report, Frank was the only child of a mother who was determined that he become a priest and of an unaffectionate father. He entered a seminary and a Trappist monastery but was forced out because of homosexual activities.

At age 23, he was first convicted of child molesting and spent about five years in a Missouri state hospital on that and a subsequent offense. Later he served 18 months in the Missouri state prison for another offense before coming to California in 1973 to escape arrest for kidnaping in East St. Louis, Ill. A 1974 kidnaping and molestation conviction resulted in his being sent to Atascadero state hospital. He was released six weeks before Amy Sue's murder and was not arrested for the crime until a year later, after he had been identified as a suspect in two subsequent kidnaping and molestation cases which seemed to fit the pattern of Amy Sue's murderer.

Frank appeared at his trial to be an articulate, intelligent man who had been studying for a degree in sociology at California State University Northridge when he was arrested. He wore three-piece suits, a well-trimmed gray beard and looked, in the words of prosecutor Prager, "just like one of us." But the jury was convinced by detailed scientific evidence showing that a pair of vise-grip pliers found in Frank's home was the same tool that left cross-hatch marks.

One of the investigators in the case pinned on his bulletin board a quotation from Frank that seemed to sum up his attitude: "I'm a good clean child molester." Prager said this light-hearted approach to the practice of sexual contact with children persists among many individuals.

Most state courts operate on the principle that molesters can be cured--or at least are not enough of a threat to society to merit long prison terms. After the judge in the Amy Sue Seitz case sentenced Frank to the death penalty, he remarked that he had little hope the man would actually be executed, or even serve a life term.

Prager said: "I expect he will get out in about 15 years for good behavior. After all, he won't be molesting any kids while he's in prison."