I think you missed the point.
Games don't give nightmares, the way they are perceived do. It's the job of us adults to show how nightmares work and how to fight them so the kids don't suffer any psychological illnesses later.
Try playing Amnesia on a bright summer day, outside while playing "Don't worry be happy" as background music and tell me again that it is a game giving nightmares. And let's have a look at all those people who grew up with Wolfenstein and Doom, some of them even posted in this forum, go ask them if they had a bad childhood and developed to be angst-ridden psychopaths cowering in fear or craving for blood.
Kids react totally different to death, suffering and horror than adults do.
You might want to inform yourself about that, it's quite revealing.