WASHINGTON (AP) _ Reports of child sexual abuse are leveling off nationwide, but in California, site of a sensational day-care molestation case, reports rose 113 percent during the first six months of 1985, according to statistics released Wednesday.

Nationally, child sexual abuse reports increased 27 percent, a slight decline from the 35 percent increase last year, the National Committee for the Prevention of Child Abuse said. The California figure was the highest of 39 states surveyed.

The large increases in California can be traced to public awareness of the problem since the founder and several teachers at the McMartin Pre-School in Los Angeles were charged with more than 300 counts of rape, sodomy and other abuse, said Anne Harris Cohn, executive director of the Chicago-based committee. Two-thirds of the charges have since been dismissed.

In Sacramento, Paul Crissey, director of the California Consortium of Child Abuse Councils, said media coverage of the McMartin case as well as television productions like ''Something About Amelia,'' a TV movie about incest, attributed to more reporting.

He also said California has strong child abuse laws ''requiring almost everybody who works professionally with children to report every case of suspected abuse.''

Georgia also had a staggering 102 percent increase in reports of sexual abuse of children, which Bonnie Engle of the committee's Atlanta office traced to a statewide campaign saying to children, ''It's OK to tell.''

At the other end of the spectrum, Nebraska had a decline of 13 percent in reported child sexual abuse.

Cohn said surveys indicate one of every four or five girls is sexually abused by the age of 18, while one of every 10 or 11 boys fall victim to molesters.

The sexual abuse figures were gleaned from general child abuse statistics compiled by the committee through a telephone survey of its 61 chapters across the United States.

Nationally, the total number of abuse and neglect reports was up 9 percent, compared to 14 percent a year earlier. In 1984, the committee reported 1.2 million cases of child abuse. It projects 1.3 million cases for 1985.

''It's a staggering number and it's staggering to think about,'' Cohn said, particularly since as recently as the early 1970s experts estimated there were only 60,000 cases of child abuse or neglect each year.

But because of the leveling-off of reports, she said, ''One may speculate that more than the tip of the iceberg of this devastating social problem has finally been recognized and made public.''