it worked out this way for most of the supernatural plots.as soon as it was over everyone acted like it had never happened and in many cases never spoke of it or appeared to have any recollection of it whatsoever.
it's interesting to note that many of the main characters besides julia and willie actually did "know" barnabas' secret...and then convieniently "forgot" it when the plot dictated.maggie knew.vicki knew.carolyn knew.david was highly suspisious and certainly on his way to knowing.but the plot dictated that all these characters "forget" it or at least have their attention otherwise focused.
For quite some time I've meant to post a link to this review:http://www.imagesjournal.com/2002/reviews/darkshadows/
It's one of the best DS reviews I've read, and the first paragraph eloquently expresses this issue
Some reviews of possible interest