I have seen it happen all the time. Children play for too long and end up with hurt fingers and thumbs. Once I even tripped over my Wii cord. If you meant mentally then I don't think it's a problem. Violent things have been around for a long time. If you think that video games are too violent, then what about movies or comics. This is what I really believe. The first part was me trolling.
All empirical and scientific evidence shows that video games do not seriously harm children in any way shape or form. No studies have been able to prove violent video games make kids more violent, which is a claim that has even reached the United States Congress more than once. Video games really are okay.