Only 36 percent of the child victims file charges, a recent study said. According to a report published by the Sunflower Children's Center from 2004 to 2011, the families of only 647 or 35.9 percent of all 1,802 child victims who consulted the center filed charges against the offenders.

The families of 37.8 percent took no action, and those of 16.8 percent said they were considering filing charges but eventually did not, according to the center.

The center is supported by the Ministry of Gender Equality and Family.

"In many cases, the parents of sex crime victims give up due to the difficult procedure of filing charges where young victims have to testify repeatedly," said Woo Kyung-hee of the center. "Another reason is that most families are reluctant to make their children's cases public."

Meanwhile, children under seven account for 51 percent of all sex crime victims and those aged between seven and 13 for 43 percent.

Some 39 percent of the child victims were sexually assaulted once and 39.4 percent more than twice or repeatedly, while 21.4 percent suffered unknown numbers of sexual assaults.

"Many sex offenders target young children, who can't express themselves well and slip through the legal dragnet even after committing their crimes," said Chung Woon-sun, a professor of pediatric and adolescent psychiatry at Kyungpook National University School of Medicine. "We should make sure that children are never left alone because neglected children are vulnerable to sex crimes."