On Youtube, there's a whole bunch of videos of kids seeing the "No, I am your father" scene from Empire Strikes Back. Some of them are pretty great.
I like the "Elders react to" series of videos. Some of those old people are awesome. The GTA-V one is hilarious, they love the game.
I don't get it. If she's watching these for the first time in chronological order, she shouldn't know who Luke or Darth Vader are. If she's watching in production order then she already knows the surprise and knows that Anakin is Darth Vader and Luke's father. If she is shocked that Anakin turns to the dark side, she really shouldn't be. He did murder the village of Sandpeople in episode 2 and then executed Count Dooku earlier in the film she's watching.
She's a kid. Hell my kid drives me crazy like that. We can be watching a movie and she'll be asking questions that make me ask her what the heck is she watching? Just ten seconds ago X happened and it's like she didn't even notice. I could easily see a kid see Anakin turn into Darth Vader and completely miss that Luke and Leia are his kids.
Meh. Both of those are excusable in context. The sand people kidnapped and tortured(?) his mother, resulting in her death. That was their while deal, they were surely going to do the same to others if no one stopped them. They had it coming. Dooku was an extremely dangerous enemy, even with his hands chopped off, and his execution was ordered by the Chancellor himself. Anakin was just following orders. Of course both incidents look worse in hindsight, but if you're watching them chronologically you wouldn't have that.