"Sex Play" or not?

    During a New Years dinner my 5 year old daughter and her 3 year cusin (boy) were witnessed in her room both naked with her on top of him. My sister inlaw is the one that saw the nudity and yelled for my 5 year olds mother to "Get Up Here". I was the first one up the stairs and when I entered the room the children were seperated and both white as ghost's. The boy kept repeating "sorry" and my daughter could barly speak. It's my opinion that she knew that somthing wrong had just happend. After my nephew and his mother left the room my wife and I both tried to speak with our daughter about what just happend and all she would say is " I don't know ".
    It's been a few days since this occured, My brother and I have talked for hours on the subject with both of our wifes yet to speak to each other. Both children have opened up and neither of the stories match. My wife and I have decided not to press this issue with our little girl but all parties have one big question. How was this learned? Both children have not been exposed to pornography or for that matter as far as we know, never seen mom and dad engaged in sex.
      For the past couple of days I have done a lot of reserch on this subjet and come to the conclusion it's normal exploration but am having a hard time getting the info to my sister inlaw. We have seen no changes in behavior from our girl but my nephew has shown some negative behavior. As a dad I feel like I've just been run over by a truck and don't now where to turn for advise.  

[Kevin Kennedy, Ph.D.]

So-called sex play is not uncommon at the children's ages. The single source of worry is this situation is the nature of the activity. That is, the apparent simulation of sexual intercourse. That does bear some continuing investigation because it does not spontaneously occur to children to do that. On eof then suggested that activity based on some awareness.