I'm not sure whether violence in videogames has as severe an effect on children than these studies attempt to imply. The studies state what effects violent videogames can have, but say nothing on the likelihood of these effects taking place. Personally, I think as long as the current videogame ratings are followed then violent videogames should not have any unwanted effects. An 8 year old child should not be playing a 17+ or 18+ rated game because his or her ideas of right and wrong are still forming and so he or she is very impressionable. However, a 17 or 18 year old's moral compass is much more developed which allows them to distinguish between what is okay in a videogame and what is okay in real life.
If I had to venture a guess as to why certain children become more aggressive through playing violent videogames I would say it is due to parents not respecting the age recommendations placed upon them. Based on personal experience, I would say the majority of parents allow their children to play videogames that are meant for much older youths. For example, I know many children very close to the age of ten who play videogames such as Call of Duty which is rated 17+ and revolves largely around shooting others in a war-like environment in order to make progress in the game.
So based on my premises, if parents enforced the age recomendations put on videogames, these studies you have referred to in your essay would likely have different results. For example, I would estimate that the effects of increased aggression in children would be significantly reduced.