There were several studies or researches that have confirmed that an ineffective monitoring of your kids concerning video games can make the kids to become very addicted to video games and it could have negative effects on them when they are too addicted to playing of video games..
Video games makes the kids happy because they find it interesting and entertaining too , so if your kids love playing video games it is a good thing as long as it does not have negative effects on their studies etc.
But addiction to video games can make someone kids to have less concentration on their studies because the addiction to playing video games might make them to use all their time to play games even during the time when they are supposed to be studying or reading..
So allow your kids play video games when it is the time to play but always teach them to understand there is time to play and there is time to study... When it us time to study they should learn and not play video games during that particular time..