P.G.
A polite hello is always appropriate. I assume that you are talking about teenagers who are old enough to be somewhere else on their own. Why should the children have to hang out with the guests of the parents for an extended period? They are not the kid's friends. What is interesting and entertaining to the adults would probably be flat out boring to teenagers. It's polite to say hello, perhaps spend a bit of time being nice to mom's/dad's friends, but unless it's some kind of family party, it doesn't seem necessary to make the kids hang out unless they want to.