How do I teach my 4-year-old daughter not to kiss boys on the lips?
First, don't freak out. Children's sexual behavior can be upsetting to us, but it's usually innocent exploration. She has seen people do this in the media and has had no indication that it is not usual and acceptable behavior. Gently remind her that this is something grown-ups do in private and that it is not for children.
This is also a good lesson for adults about the profound effects media can have on children. Look how easily her behavior was influenced by what she saw. That is why we encourage parents to limit television time and make sure programming is appropriate for the child's age.
On the other hand, when children exhibit a preoccupation with sex it may be a red flag that there is something going on in their lives that they are not ready to cope with. Children who are exposed to pornographic material or who have been sexually abused may behave in the way you describe. If your daughter persists in this behavior, see your pediatrician.