Parenting and children's sociosexual behavior
We are often accustomed to thinking that sexual behavior begins during adolescence. [...] As the pleasure circuitry of the sexual response is partly developed early in development, young and older children may engage in increasing rates of autostimulation and sex play. Infant boys may display erections, infant girls are capable of vaginal lubrication, and both are capable of orgasm, although without ejaculation. During mid-childhood, more social sex play, with same- and other-sex partners, becomes more common. As kids around 6-7 years of age are more independent and increasingly engaged in a wider social world of other kids, sex play can be viewed as a common aspect of social development.
source: Article 'Parenting and Children's Sociosexual Behavior' by Peter B. Gray (Ph.D); www.psychologytoday.com/blog/the-evolving-father/201304/parenting-and-children-s-sociosexual-behavior; Psychology Today; 17 April 2013