Ah, this reminds of when I was a student and working at Game. GTA Vice City had just been released, and the number of parents buying it for very young kids was shocking. We were instructed to remind them that it was an 18 rated game, and to warn them of the content. Some just shrugged, others were horrified.
My favourite was the mother who looked confused and said, "I thought the age rating was difficulty, it's just racing cars, isn't it?"
I explained that the player character is a criminal, and about the murder, theft, drug dealing, prostitution, gang violence, organised crime and strong language. She looked at me, looked at her 8-year old son, and said to him, "Timothy! You told me it was a racing game!"
Timothy ended up not getting Vice City that day.
By all means, allow your kids to play 18 rated titles if you feel they're suitable - games ratings tend to be harsher than their movie equivalents. I played Mortal Kombat as a young teen, and had watched the Terminator, Alien and Predator movies before then.
That said, graphics are much more realistic now - I'll probably be a lot stricter in that respect than my own parents were, and I say that as a massive gamer (I make games for a living!).
Just be aware of what the games actually are, and don't try and fool yourself that any kids are playing GTA just for the racing.