I'll basically be talking from anecdotal evidence and a general reckoning on the subject, so take it all with a pinch of salt.
For the most part I don't believe that media violence as a whole (particularly video games which take the brunt of the blame) is responsible for creating violent children.
I won't deny that it can desensitise children to violence, but that doesn't necessitate children imitating such violence.
I don't know the statistics for such cases (particularly because records don't go back so far) but I'm pretty sure there was equal, if not more cases of violent children prior to media violence becoming prevalent (I'm talking pre-TV, pre-film, pre-Radio).
I would say that there probably was more violence in children in this age, and that the cause of it was poverty, poor parenting (should it even exist as orphans were much more common due to social/medical reasons).
There's no doubt that a child who has access to violent media (whether games, newspapers, TV programmes, etc...) may be inspired by the depictions in that media, but that's no different than hearing of violent acts in books or poetry or in general discussion.
Bouncing off of Headcrab Zombie's post young children exposed to such things may be more inclined to imitate out of innocence, but that imitation could be of anything.
I think that there has to be a predisposition towards violence in a child, whether a result of mental instability, psychological trauma, or simply a case of a complete lack of violence. Blaming violence in the media is clutching at straws for a reason when you simply don't want to blame someone responsible, whether it's the child, the parent or the state.
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