If a child is sexually abused, the best action, experts advise, is to report the abuse to the authorities and get the child prompt medical attention to minimize the risk of contracting a sexually transmitted disease. Most important, parents need to provide unquestioning love and support to a youngster or adolescent who has been victimized. Studies show that those are the children who recover best.

Among the programs and agencies providing help -- including treatment for victims and offenders -- are:

*Child Sexual Abuse Program, Division of Child Protection, Children's Hospital National Medical Center, 111 Michigan Ave. NW., Washington, D.C. 20010; 745-4100. 24-hour emergency treatment for sexually abused children. Also runs a rehabilitation program for adolescent sexual offenders and publishes pamphlets on sex and child abuse.

*Chesapeake Institute, 10605 Concord St., Suite 105, Kensington, Md. 20895; 949-5000. Offers counseling to child and adolescent victims of sexual assault and to adults who were assaulted as children. Also provides treatment to sexual offenders. $60 a session.

*FACT -- Families and Children in Trouble, local chapter of the National Committee for the Prevention of Child Abuse, 1400 20th St. NW, Suite G-100, Washington, D.C. 20036. 628-FACT is a 24-hour hotline providing information and referrals for child sexual abuse and physical abuse reports in the District, Maryland and Virginia. Also offers resources for immediate help to child victims and former victims -- including cases of incest -- and provides free information for parents about child sexual and physical abuse.

*Johns Hopkins Sexual Disorders Clinic, 600 N. Wolfe St., Baltimore, Md. 21205; (301) 955-6292. Confidential treatment for people suffering from sexual disorders offered on a sliding scale fee.

*National Committee for the Prevention of Child Abuse, 332 S. Michigan Ave., Suite 950, Chicago, Ill. 60604; (312) 663-3520. Publishes several low-cost booklets on child sexual abuse. Catologue free upon request.

*National Center on Child Abuse and Neglect, P.O. Box 1182, Washington, D.C. 20013; 245-2840. Gives information on various forms of child abuse treatment and prevention.

"Talking With Your Child About Sex," by Dr. Mary Calderone and James Ramey (Ballantine Press, 1984; $2.95).