I know that many will say that GTA is just a game, but, again... i will try to bring a different point of view.
Kids, in general, needs limits. Kids constantly challenge their parents to see how far they can go before get a punishment, and this is natural. It's part of the psychological growth. So, someone has to tell the kid to stop somewhen and who are the responsible to do so? Their parents... so, that's basically why parents tell you no to a bunch of stuff.
Now, GTA... as much as we, gamers, know that real life and fiction do not mix, kids do not go around killing other people just because they did that in the videogame. That's absurd. But, as much as we like GTA, that game has in it's core: Extreme violence, pornography, drugs, stealing, lieing, racism, prostitution and a lot of other bad things. Sure, it is just a game, it is not real and nobody will go around using drugs just because the GTA character did so.
But, it's not because that your kids is smart enough to recognize that what is right or wrong, that it is ok for the kid to be in contact with this sort of thing at least 1 hour a day. A rough comparision would be, is it ok for your 14 years old kid watch 1 hour of pornography on internet? I am sure you don't think so. Or 10 years old kids looking at nude pictures for hours. Do you really think that sounds remotelly ok?
So, it's not about "censor" it to the kids just by the pleasure of censor things, it's a matter to stablish a limit, to anything. Be it a videogame or not. Kids are not supposed to have so much contact with this sort of thing, and we all know that. That's why we don't swear on this forums, this is why we don't post nudity on this forums, that's why we have to keep it PG13.
And to make it sure, if my kid ever come to me, and ask to know about drugs, pornography or anything else, i will sure tell them. Because that is how you teach kids, telling them and giving them some guidance, not allowing them to do whatever they want. As i said before, communication is the key, for almost everything.