In order to know if a child’s behavior is of concern, we need to first understand what is “normal” sexual behavior for the child’s age.
For example, looking and touching of both same and opposite gendered playmates is fairly normal; however, anything oral-to-genital, genital-to-genital, or insertion of anything into the genitals is not normal.
Children rubbing or touching their genitals is rather common. However, a child who will not stop this behavior when asked, or rubs/touches to the degree where they cause injury to themselves, is not normal.
A simple, straightforward, but comprehensive, resource for both parents and professionals is Expanded 2015 Understanding Children’s Sexual Behaviors by Toni Cavanagh Johnson.
Source: Know What is Normal, Children’s Home Society of South Dakota