Schools and social services are ignoring warning signs over the sexual behaviour of children who later go on to abuse other youngsters, probation inspectors have found.
Around one in three of the children investigated by inspectors had shown concerning behaviour that was ignored by authorities before they went on to commit sexual offences. In one case, a boy who was first convicted of a sexual offence at the age of 16 had been displaying inappropriate behaviour since he was eight years old.
Inappropriate behaviour was "too often subject to disbelief, minimisation and denial by professionals as well as families and treated as a one-off," Her Majesty's Inspectorate of Probation (HMIP) said.
The inspection visited six youth-offending teams in Tower Hamlets, Sheffield, Cornwall, Birmingham, Lancashire and Vale of Glamorgan.
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."
Ms Calderbank added: "This, to us, represents a lost opportunity, both for the children themselves and their potential victims."
Questions were not being asked why the children were displaying sexual behaviour, some from quite a young age, the inspectorate said.
Addressing the reasons why authorities or parents did not act, assistant chief inspector Julie Fox said: "We don't want to believe that a child could do something like that."
In addition, inspectors found that cases were slow to get to court and took an average of eight months between the offence coming to light and sentencing. This resulted in long periods in which no work was done with the offenders to reduce risk of reoffending, HMIP said.
Around 12% of sexual offences are committed by children aged between 10 and 17 years old.