Here's the deal with kids. They don't see or necessarily even know what to expect if not told. Something that we, as adults, don't even give a second notice too, can have them mesmerized. So you are correct, kids will come to their own conclusions about whether or not they like something.
Here's the deal with parents. Especially parents that love Disney. They desperately want to have their kids love it as much as they do, so they will pre-sell it and then when things don't go as planned have to backtrack on what they said and cover it over so it didn't sound like they were just BSing the kid. It's all very innocent and well intentioned and a natural way that parents usually act. No one has done anything wrong, really (other than Disney for letting their fans down) Parents should be able to tell their children what they love and want them to feel the same way. It's an important form of bonding. They should also do whatever they can to not jade the young mind by overreacting negatively to something that disappoints them. That's not for everything in life, but, it certainly does apply to a theme park.