Children and the Media - Your initial idea

Good idea to use vide games for the basis of your research, but not a good idea to try to prove a causal link between them and violent behaviour, because as you will find out, it is impossible to prove such a causal link. Behaviour is determined by many many factors and you cannot separate out one of the as the cause.

However, you COULD look at how violence operates within the narratives of video games, and how children talk about video game violence. You could explore the areas they seem to be learning about through these violent representations.
Sean