I think one of the reasons LDS girls get knocked up is they think they're not going to have sex, but then let themselves get into a situation where they end up doing it. Whereas, kids who don't think they need to wait until marriage to have sex may be more likely to come to such event prepared, because they've been hoping all along they were going to do it.
So, my advice is to tell your kids to pack a couple of Trojans in their scriptures and then to make sure they keep their scriptures with them whenever they go out.