My parents were also nice in not giving anything away when fisrt sat down one boring summer day with these rather old looking tapes. It was around 1994 or 1995 when I was in maybe 2nd grade. When the reveal came, I had no clue about it. I never heard much about Star Wars in my life before that to really care. I was totally screwed up because I'd never been presented such a downer movie ending in my short life. I never actually thought he was lying about it because it just seemed to work for the story (which probably comes from first watching all of them on the same day with ANH still very fresh in my mind).

But now a whole new generation will have these movies shown to them I-VI and unless they are totally unable to pay attention for the PT (thanks to it being too complicated and disjointed for the average 6-7 year old) then the Vader reveal won't strike them as anything special, just another thing that you know but Luke doesn't (because it's not about Luke right........).