I saw SW and ESB in the theaters when I was 17 and 20 years old, respectively. By the time I saw ESB in the theater, everyone already knew the reveal -- the secret lasted one day, if that.

Having said that, the reveal is significant and is meant to be a surprise. That is why they must be seen in the order 4,5,6,1,2,3. And for that reason, there is no reason to hide the reveal in the prequels.

Furthermore, it would be impossible to hide the reveal... when Ben talks about Anakin being a good friend that he was tutoring, who ELSE could he be talking about? He also says that Vader helped the empire destroy the Jedi. In ep.3 we watch Anakin do all those things, and then become Vader. You'd have to ditch pretty much all of ep.3. Hmm, not a bad idea, now that I think of it.