I'd let my kids play whatever the hell they want! Video games, no matter what the genre or theme, make excellent learning tools. When I was five, my dad let me play Return to Castle Wolfenstein GOTY edition, and because of that one game, I have a great interest in the Germanic language and culture, my hand-eye coordination, and I know how to quickly reload a WWII era Luger, should the need ever arise. Plus, I've gained an impeccable vocabulary, healthy and active imagination, and a great social life. All of which, I got from playing, analyzing, and thoroughly enjoying each game I played.
However, I'd have to do some reassurances. First, I'd have to make sure the game didn't have excessive sexuality, language, or racist themes. Second, I'd have to talk to them before they play their first game, and make sure they realize none of what they see is real (Unless the game is by History Channel). Third, I'd want them to know that swearing doesn't make someone mature, it just makes them sound stupid.