I don't know if it's so much the "pre-show" as the dark part of the ride before it starts. The storyline is about a group of people (including at least 1 young child) who are on the elevator when it gets struck by lightning. They disappear into the Twilight Zone, but show up as ghostly images a few times. (I haven't been on since the new drop sequences, so don't know how it's different now.) You ride through dark rooms, with weird images from the Twilight Zone show appearing before you, along with creepy music. A Rod Serling sound-alike talks about how the same thing is about to happen to you.
What gets my daughter are the bellhops. They act very somber, and will say things about how you're not likely to come back, etc. For some reason they really scare her.
When she was 4, we took her on the ride and she loved it. She even teased mom for being afraid of the drops and screaming so much. Next trip (4 1/2), she rode it once, seemed okay, but said once was enough. Last time (5), she didn't want to ride. I think that some kids get more scared as they get older, as they become more able to understand why things would be scary and their imaginations develop.