kissing "games"normal? how to talk about it?

[ellairiesmom]

DD1 is 3, DN is 4 & a different DN is 7.  We keep catching them kissing.  Sometimes it goes along with dress up/pretend play, mommy/daddy play, princess play but it has also happened just while watching a movie or TV.  Recently there was an incident where the oldest laid on top of the other 2 & kind of "made out".  SIL (mama to 4 yo) walked in on that & freaked out on 7 yo (she was on top) who immediately cried.  It was clear she knew she was doing something wrong & I also know she was talked to about this last year when it happened with the 2 of them another time when we weren't there.  We all talked to them about not touching other people & that kissing was not for them.  There has never been any clothes off-it seems to be just kissing.  

I remember doing this stuff when I was younger, mostly with neighbor girls. 

Is 7 considered so much older to 3 & 4 that I need to look at it like a predatory thing?  It has happened without her being present too.

How do I explain to our loving & affectionate 3 yo why it's isn't ok to play "pretend kissing"?  

Especially when she says goodbye to people, she often kisses on the lips.  So we are telling her she can't kiss on the lips under certain conditions, but it's ok for saying goodbye?  

I also think that with a 3 yo, the more you talk about not doing something, the more they are interested.  I am inclined to think a 4 yo & 7 yo are more in the category of knowing what they should & shouldn't be doing etc.  Am I wrong here?  

I just want to know the right things to say & how to handle it in a healthy manner but I also want it clear that they should not be doing this & in no way is it ok for my 3 yo to be doing it.