Many sex crimes by children 'missed' 7 February 2013 Last updated at 03:45 Some social workers, teachers and police fail to tackle sexual offending by children and teenagers because they miss signs of inappropriate behaviour, according to a report. Probation inspectors in England and Wales found inappropriate behaviour was "too often subject to disbelief." Government officials said they were "driving up the skills and experience of social workers." The report focused on 24 teenage boys who committed crimes against children. Inspectors visited six youth-offending teams in London's Tower Hamlets, Sheffield, Cornwall, Birmingham, Lancashire and Vale of Glamorgan. They found that opportunities to intervene when the boys were younger had been missed in nearly every case. Chief inspector of probation Liz Calderbank said: "The most shocking finding was that a third of cases had come to the notice of the authorities - usually either schools or social services - previously. These concerns were not acted on, dismissed, minimised or ignored. This, to us, represents a lost opportunity, both for the children themselves and their potential victims. Rapes and assault Explaining the factors that sometimes led to abuse, she said: "There were elements of coercion involved, there was clear power imbalance between the individuals and there's evidence of grooming." The inspectorate said questions were not being asked about why the children had been displaying sexual behaviour, some from quite a young age. The report criticised the "unacceptably long" delays in bringing cases involving child sex offenders to court - on average eight months from allegation to sentence. It said that during that time there was usually no work done with the alleged perpetrator and no support for the victim. About 12% of sexual offences are committed by children aged between 10 and 17 years old. The cases of 24 offenders aged between 13 and 18 were examined, covering offences ranging from indecent assaults to rape. All but one of the cases had issues that related to physical or mental health, while seven had previous offences. And 21 had concerns relating to the family environment. Government officials said they were "driving up the skills and experience of social workers" so they were better able to identify warning signs.