At one point in my childhood (age 9 or so) I would cry at any remotely sad film, including Tom and Jerry.
Joe's point about unreality is valid, in that knowing it's only a story can remove the element of fear. However, watching a character being mean to another character still reminds me that there are real-life examples of similar meanness happening all the time. I suppose it also reminds me of upsetting experiences that I have had in the past.
I know plenty of other people (children and adults) who can be moved to tears by a film. I believe it is a continuum: very few people are totally emotionally unaffected by sad stories, most people are slightly affected, a few people are ridiculously over-affected.
To function in society, I learned a few strategies:
- control the tears so that companions are not aware of my feelings
- divert my thoughts quickly to something else rather than dwelling on them
- avoid films that are gratuitously horrible
I still fail to understand why anyone would choose to watch horror movies. If it is scary enough to cause fear, then why would you want to experience that? If it is not actually being scary, then it has failed and what was the point?
Other than that, I'm doing OK, and I'm sure you don't need to worry about your niece.