Or maybe she knows part of the truth but not all of it? Maybe since they are around 15-16 not all his powers have come through yet. Maybe he shows her and his family a few but as they get more and more strange he hides them and just doesn't use them in an attempt to be more normal... after all they can all say he is just a Russian experiment if it is just a few funny things... but after he starts flying the alien theory starts to be more feasible and you wouldn't want to freak out your girlfriend and tell her she may be having an alien baby now would you?
There are many many ways to string this so that you can still have a revelation of sorts with out Lois being played a fool.
And as with the kids... we have seen many stories where the kid didn't know till much later. I'm just wondering why everyone is picking both of them.
