A Christchurch agency that treats sexual offenders is now helping children as young as five.
STOP Trust manager Don Mortensen told The Press the age of children the trust worked with had got lower over recent years.
Mr Mortensen said the organisation did not call what the children did "sexual offending, it's sexually inappropriate, but some is quite disturbing."
He said one girl had abused a boy at school for a whole year, she would strip him and assault him.
Mr Mortensen said there was no one reason why more young children were acting inappropriately, but they knew more at a young age than in the past.
He said children were being exposed to explicit adult material, which meant they were a lot more sexually aware.
Adolescent services clinical team leader Maureen Lorimer said it was the demand from schools that prompted STOP to set up the children's programme three years ago.
She said there had been a lot of concern from principals and parents about how to manage safety within a school.
The STOP Trust has run adolescent programmes for 15 years but it is only recently that the age of children demonstrating inappropriate behaviour had plummeted.