Anakin falling to the dark side and becoming Darth Vader (if you watch the saga in order). When I was 6, I like most in my generation was blown away by the reveal in ESB. I didn't even believe it until Yoda confirmed it 3 years later in RotJ. For my nephews, I showed them the saga 1-6, and not only were they in complete denial when their hero becomes the villain in Ep3, they still went nuts when Vader reveals the truth to Luke in ESB. They had been freaking out for a week after ANH trying to figure out how/when Luke would discover the truth.