Talking to children about sex has always been embarrassing to parents and kids alike. As a result, there's very little conversation in many families despite the fact that children are becoming sexually active at younger and younger ages.
A prepubescent child is not a sexual being. 'Sexual' behaviour such as touching genitals could be due to mere curiosity about their anatomy-nothing to worry about. In cases where such behaviour recurs too often, actively mimics and or attempts to have sex- then the parent should be very worried.