"That was probably the darkest week of my life without a doubt," explains Claire.
"All the anger and the rage, the confusion, the revulsion. I couldn't look at her. I wanted to grab my son and run and never see her again. I was just terrified."
But Claire had more to face when it quickly emerged that Hannah had also been sexually abusing several other children.
Hannah is typical of one third of the children who display sexually harmful behaviour - to use the preferred clinical terminology - in that she has also been sexually abused herself.
Hannah was regularly sexually assaulted by a teenage boy for around two years.
"She didn't understand at the beginning what she was doing was wrong. For her it was just learnt behaviour," explains Claire.
"Her interaction with other people was all based on that because it went on for two years. So that can't be wiped away, we just have to steer her away in a different direction."
Wesley Neailey was killed by a teenager with a history of abuse
Claire says that finding out what had been happening has had a devastating effect on her relationship with her daughter.
Initially, she says, she even questioned whether she would be able to love Hannah again and rebuilding trust in her little girl is still proving hard.
"Even after all this period of time has passed I still worry, what's she doing? I worry when she's at school. Is she being monitored? Is everything ok, are other children safe around her and what will the future be?"
It is a startling fact that one-third of those who sexually abuse children are just children themselves; something which challenges our notions of childhood and understanding of what children are capable of.
Dismissed as play
Claire says that her experience has made her realise just how taboo the subject is with people, most of whom prefer to think that what is actually sexually harmful behaviour is just normal childhood playing.
FIND OUT MORE
Jackie Long also reported on this issue for File on 4 on BBC Radio 4 on Tuesday 16 March 2010 at 2000GMT, repeated Sunday, 21 March, at 1700GMT. You can listen via the BBC
or download the
Even professionals, she says, questioned whether she was misinterpreting what her daughter had been doing.
"If I hadn't been thrust into it in such a horrible way, I wouldn't have wanted to believe it either," she says.
"But this wasn't kiss chase. This wasn't something innocent... it was a sexual act. It wasn't role play in a childlike way.
"It was something you might imagine seeing in a late-night porn show."
Hannah and her family are now receiving treatment with a specialist counsellor at a project run by the charity Action for Children.
But the experts who work with children like Hannah say many of them are not getting the help they need to stop.
The systems in place to deal with this problem are often so inadequate that child professionals either do not spot sexual abuse being carried out by children or - perhaps even worse - they recognise it but do not know what to do about it.
Consultant child and adolescent psychiatrist Dr Eileen Vizard works with many of the most disturbed young people in Britain and is a leading expert on children who display sexually harmful behaviour.
In the past, she worked with one of the boys who sexually assaulted and murdered James Bulger.
Our research shows that they may wait on average four-and-a-half years before they come to be seen by us.
Consultant child and adolescent psychiatrist Dr Eileen Vizard
By the time young people get to her unit they are extremely dangerous sexual abusers.
But she says that in many cases their problematic sexual behaviour has been spotted by other professionals years earlier, who through panic or poor training, either ignore it or mark it down and pass on the file.
"Our research shows that they may wait on average four-and-a-half years before they come to be seen by us. Often that child's case will come in with a huge amount of associated background information.
"Sometimes there are boxes of meticulously written notes which I would have thought that anybody reading would think 'this isn't too good, why don't we do something?'"
Dr Vizard is not alone. Several specialists have said they are equally frustrated by the lack of action.
One doctor - who did not want to be named - said it was not uncommon for a serious teenage sex offender to be referred to him, complete with a detailed chronology marking down instances of very problematic sexual behaviour dating back to when he was three or four years old, but nothing had been done.
Inconsistencies in the way young abusers are dealt with are also a concern.
Prof Simon Hackett says there is a postcode lottery of care
Professor Simon Hackett, whose work at Durham University focuses on child welfare, conducted a survey of services available across the UK and the Republic of Ireland.
Forty percent of respondents said they had no access to any kind of assessment service in their area, and only 8% said they were satisfied with the availability and quality of intervention services.
Prof Hackett says that effectively what exists is a postcode lottery.
"Three 13-year-olds in different parts of the country behaving in the same way - one of them may get no service at all and the family might be left on their own struggling with how to help that child.
"The second child might be given a child in need service routed through the Child Welfare Service, and the third child maybe convicted and sent through a criminal justice system."
Failure to act
For several years there has been widespread recognition, even within government, that the system is not working as it should.
They've let me down, they've let my dead son down and they've let other children down
Liz Neailey, whose son Wesley was murdered
As far back as 2003, former Lord Chancellor and Justice Secretary Lord Falconer acknowledged it was an area which had been neglected and there was a need for a national strategy.
In 2006, the then Prime Minister Tony Blair, said such a strategy for England could be expected by the end of that year.
It was again expected by the end of 2009, but the Department of Health have now pushed that back to the coming summer.
Dr Vizard, who contributed detailed submissions to a draft strategy around five years ago, is mystified by the delays, but certain of their impact.
"I do think the main cost has been with the victims who have been created by these children and adolescents who have not been able to access good services because whilst they have not been helped... more victims have been created, [and] have suffered a very great deal."
Slipped through the net
One woman who knows that from bitter experience is Liz Neailey. Her 11-year old son, Wesley, disappeared from his home in Newcastle in June 1998.
Dominic McKilligan's abusive past was known by various agencies
Wesley's body was found a month later. He had been battered with a wrench and strangled.
His murderer was 18-year-old Dominic McKilligan. He had a string of convictions for serious sexual assaults against young boys dating back to when he was just 11 years old - all known to a variety of different agencies.
A review found a litany of missed opportunities and attacked the authorities for failing to work together, effectively letting McKilligan slip through the net.
Among its key recommendations was an urgent need for a national strategy to deal with young sex offenders, but nearly 12 years since her son's death Mrs Neailey is still waiting.
For her the delays are quite simply a betrayal.
"They've let me down, they've let my dead son down and they've let other children down. When are they going to take things seriously?
"We need these strategies put in place, not just to help my children, but your children and your grandchildren too."
Jackie Long also reported on this issue for File on 4 on BBC Radio 4 on Tuesday, 16 March, at 2000 GMT, repeated Sunday, 21 March, at 1700 GMT. You can also listen via the BBC iPlayerafter broadcast or download the podcast.
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