I am a pretty strong Atheist but my fianc�e demands for our children (when we have them) to be Christened.?

This is a non-issue. If there is no God (or even if there is) what harm is there in having them Christened. Why don't you wait until she's pregnant to decide about that. In fact, why is it even an issue or non-issue, as it were? The key to a successful marriage (one where one of you dies before getting divorced) is compromise and in many cases surrender.