Over the last two years, David forced the girl, who is now three, to touch him inappropriately several times, fondled another friend's four- year-old daughter and was caught molesting his baby sister.
David is one of an increasing number of children who have sexually abused other children. The latest research claims at least one of every 100 children is sexually abused by other children by the time they reach adulthood.
Sexual abuse by siblings is believed to be more common than previously thought. Research to be published next year, by Dr Kevin Browne, from the School of Medicine at the University of Birmingham, suggests that 2 per cent of undergraduates have been sexually molested by a brother or sister during childhood.
David's mother, convinced his behaviour was more than "normal experimentation", was afraid he would become a serial abuser, so she contacted social services. After more than 20 meetings and one-to-one sessions with counsellors his behaviour has not improved.
"He has touched his sister and our friend's daughter where he shouldn't," said Rachel, his 34-year-old mother. "When I asked him why he did it the last time he said it was because he loved her lots and lots.
"He needs professional help. I am worried sick that the next time he does something he could really hurt someone or end up in serious trouble. If he doesn't get help now he could end up being a rapist."
His mother tries to make sure he is not left alone with her friend's daughters and does not allow him to watch television programmes containing sex or violence. "He goes to bed early, and does not hang around with older boys, so I don't know where he learns it all," she said. "He is terribly mixed up. He is old beyond his years. If we argue, I could be arguing with a 16-year-old."
An analysis of the child protection register shows a third of sexual abuses against children are committed by teenagers or younger children.
Nearly 500 children were found guilty of sexually assaulting another child in 1997. Home Office figures show that the number of children under 18 found guilty of sexual abuse increased by a nearly a third in the last five years.
Experts blame the rise in child sex offenders on neglect, physical, sexual and emotional abuse of the offending children, as well as television and magazines which promote sexual activity.
Last week, a judge was widely condemned by child welfare groups for saying a gang sex assault by boys aged 13 and 14 on a 13-year-old girl was "youthful exuberance". He gave two boys 12-month supervision orders. After he said the assault amounted to a "game that went too far", experts accused him of taking a huge step back in the understanding of child sex abuse.
The girl had been playing hide-and-seek on the playing fields of a primary school when she was tripped, and pinned on the ground by a boy. Other boys started fondling her breasts and she screamed for help. She ran home in tears and told her parents, who called police.
"The limits have been so blurred that some kids can go over them without really intending to or being malicious," said Michelle Elliott, director and founder of Kidscape, a charity that promotes child safety. "The pressure of all of the magazines and television programmes is enormous. Sex is all around these kids."
But the long-term effects of such assaults should not be underestimated, she said. "It will be incredibly traumatic for this 13-year-old girl. She didn't want this to happen. It will probably affect her feelings about herself, her body and relationships with boys."
Parents who contact Kidscape often feel rage and guilt that they weren't there to protect their child, and frustration over a system that does not seem to treat peer sex abuse seriously.
Children are also sexually abusing each other at younger ages, says the National Society for the Prevention of Cruelty to Children. "[This abuse] is far more serious and widespread than realised," said Gerry Tissier of the NSPCC. "The average age children start sexually abusing others has dropped from 14 to 12 in the last five years.
"We are not dealing with little boys who lift up girls' skirts, but young people who are carrying out sustained and systematic abuse. In London alone we have nearly 300 children waiting to be treated, some five years old."
The charity plans to go nationwide with its "Young Abusers" project, which deals with children who sexually offend. The aim is to have at least one project running in every region.