Instead of saying "could a 9 year old understand" and using your neice as an example, ask yourself "could I understand what was going on?"
When I was 9, I wouldnt have been interested in the story so much as seeing Spiderman kick some ***, or seeing cool lightsaber fights. The reason I wouldnt have cared about the "story" is because, to me, at the time it would have just been an interruption between the action. It wasnt that I wasnt capable, it was that I didnt care.
And as for the Hulk, here's what I found: "MPAA Rating
PG-13 - for sci-fi action violence, some disturbing images and brief partial nudity." Not all about violence.
And just so you know, 2001: A Space Odyssey was rated G. How many kids do you think would follow that movie?
"Guns don't kill people, I kill people."