CHILD welfare workers have suggested educational dramas be produced to teach children how to avoid sexual abuse, following the revelation of a huge increase in the number of cases over the past three years. Voluntary and government agencies recorded a total of 34 cases of child sexual abuse last year, compared with 15 in 1991 and nine in 1990. Welfare workers believe the figures only showed the tip of the iceberg as their survey found that 80 per cent of women interviewed said they had been sexually harassed when they were under 16 years old. According to the survey, nearly 60 per cent of the 455 respondents - aged 21 years or older - said they had been ''unnecessarily touched and rubbed'' at least once in their childhood. Forty-three said they had been kissed against their will and 27, or 5.9 per cent, reported that they had been asked by the abusers to have sex with them. These abuse cases most often took place in public places (48.6 per cent), public vehicles (26.6 per cent) and homes (16.8 per cent). About 14 per cent of the victims said they knew the abusers. The director of a children's welfare group Against Child Abuse, Mrs Priscilla Lui Tsang Sun-kai, said she believed the figures did not ''reflect the true picture''. ''Very often the victim is too shy to tell others and sometimes the abuser is her brother or father and the child is forced to keep it secret,'' Mrs Lui said. Miss Linda Wong Sau-yung of the Hongkong Federation of Women's Centre, which has conducted the survey in conjunction with other women's groups, agreed: ''A child victim may not know she was abused until she had grown up to understand more about sex. ''Children are often the targets of abusers because they are too small to know about sexual abuse and they are too weak to resist it.'' Mrs Lui, who has attended an international conference on child abuse in the United States recently, said: ''We have heard of a case in which the victim had been from time to time abused by her father in her childhood. ''And she felt too shameful to tell others of her terrible experience until she was older.'' According to the Social Welfare Department, the number of incest cases being investigated jumped from two in 1990 to six in 1991 and 17 last year. The department attributed the sharp increase to the victims' increasing willingness to seek help. Mrs Lui suggested Hongkong follow the American example and produce educational television dramas or cartoons to teach children how to deal with abusers. ''In the United States they have educational cartoons like Spiderman that teach children to distinguish a good touch and a bad touch and what to do when facing an abuser,'' she said.