But the statistics are confronting.

Children commit up to half of all child sexual abuse, experts estimate. Half of that involves one sibling molesting another. More than 800 children are reported in NSW each year for causing serious sexual harm to other children. And much abuse goes unreported.

Dale Tolliday, a clinical advisor to the public rehabilitation service New Street, says if society learned the true rates of harm "we would be terrified".

"It's almost unbelievable for the average person, to think children would be capable of doing something like this," Mr Tolliday says.

The NSW Health social worker has called for more clinics to prevent lifelong psychological damage to victims.

But, two years after a report recommended doubling the number of clinics, the government has no plans to increase funding.

Thirteen is the average age of New Street clients. Most abuse much younger children. Many have been sexually abused themselves or exposed to violent pornography.

Yet the greatest risk factor, according to Mr Tolliday, is witnessing domestic violence between parents.

"Most of these boys are carrying quite strong misogynistic, heterosexual stereotypes about entitlement and sexual behaviour," he says.

These children go far beyond normal sexual play.

"Normal is 'you show me yours and I'll show you mine'," says Freda Briggs, a child protection academic from the University of South Australia.

Problematic sexual behaviour, in contrast, includes penetration and oral sex. It is often characterised by secrecy, bribes and threats.

"They are not easily distracted and they are angry when they are stopped," Professor Briggs says.

The Royal Commission into Institutional Responses to Child Sexual Abuse heard last year the majority of child sexual abuse in out-of-home care was likely committed by other children.

It is also a problem among children years away from puberty, as recognised by new NSW Health guidelines for treating under-10s.

Forensic psychologist Ian Nisbet says international and Australian research suggests children commit 30 to 50 per cent of child sexual abuse and 14 is a peak age.

"When you're a 14-year-old you have a lot of regular access to younger kids," he says.

Although only a small minority go on to abuse as adults, some children are already acting out violent impulses.

"I once saw a 14-year-old guy who was eventually convicted of three ... rapes," Mr Nisbet says. "He was carrying around gloves and a mask and ties in his school bag."

To convict someone under 14 of an offence, prosecutors must show the child understood their actions to be seriously wrong. Most cases never reach trial, often because the victims are too young to give evidence.

In cases of sibling abuse, many parents are reluctant to press charges against their own children or even make a report.

Public treatment services cannot begin to keep up with reported cases.

Police reports of sex offences by juveniles in NSW have increased by 10 per cent a year for the past five years.

Mr Tolliday told a recent Legal Aid conference that  more than 800 children aged 10 to 17 were identified in NSW each year as causing serious sexual harm.

The free rehabilitation services in Sydney, Newcastle and Dubbo only treat about 80 children a year.

Another service is slated for the Illawarra region. But Mr Tolliday says the Sydney clinic has three times as many referrals as places and large parts of NSW lack free treatment.

"If we had a waiting list we would never get to the end of it," he says. "We need a service in each part of the state."

A 2014 KPMG report found that after counselling and home intervention, fewer than 2 per cent of New Street children re-offended sexually.

The report recommended the government build new sites in Sydney, Bourke, Orange and Taree. Opposition community services spokeswoman Tania Mihailuk said the existing services were clearly not enough.

"The government must reassess the limited support that they are currently providing and ensure free treatment services are expanded throughout the state and properly resourced," Ms Mihailuk said..

Health Minister Jillian Skinner, whose department funds New Street, declined an interview.

A department spokeswoman said consultation for "NSW's first whole-of-government sexual assault strategy" was underway.

But government agencies say they lack data on the prevalence of child-child sexual abuse.

The NSW Ombudsman does not need to be notified of child abuse committed by a child, unless the victim has a disability.

The Department of Family Services could not say how many children were reported for causing sexual harm in out-of-home care.

"Work has commenced between the Office of the Children's Guardian and FACS to establish enhanced reporting arrangements," a spokesman said .

Jacob's story

Last year, Erin noticed changes in her four-year-old son Jacob*. On coming home from kindergarten he would hide under the bed and scream. He lost a lot of weight. One night Erin found him trying to inappropriately touch his brother in the bath.

"I said to him, 'You know we don't do that sort of stuff. You don't touch other people's parts and you don't let anyone touch yours.' And he said, 'But so and so does it at kindy'."

A boy at Jacob's rural kindergarten in South Australia had been molesting him and other children. "We didn't believe him at first," Erin says.

But then Jacob said the same thing to his teacher, who gave him a picture of a kitten for his bravery. The allegations were passed on to Erin but the abuse continued.

"They misdiagnosed what had happened to my child as age appropriate," she says. "It wasn't age appropriate. It was actually deemed quite serious and concerning. It involved penetration."

Erin says the boy with problematic sexual behaviour continued to attend the kindergarten until the end of the year and the number of alleged victims grew.

A Department of Education spokesman would not answer specific questions because of the "extreme sensitivity of the issues" but said they were taken seriously from the beginning.

Jacob now goes to school in a different town and attends counselling sessions. "Our biggest fear was that our son would eventually become a perpetrator," Erin says.

The counselling sessions have progressed well but the family has struggled with the taboo of child-child sexual abuse in a small town. "We've been outcast completely," Erin says.

She warns parents not to take a "boys will be boys" attitude and dismiss potentially grave symptoms as play.

"Don't be so naive," she says. "We were those parents."

*Names have been changed

Child Protection Helpline 132 111