My nieces, now 8 and 12 respectively, are huge fans of GTA.
While they occasionally will do horrible things like run people over and jack cars, like others have said, they mainly just love to drive different cars around the city. They love the radio stations.
This is part of the universal appeal of GTA. You don't have to play the game like a vicous criminal in order to enjoy it. Driving or flying around the city is just a lot of fun, because you have the radio on, and the streets and traffic are very immersive.
The GTA series gets a bad rap in the media. People make it out to be a hooker murdering/cop killing game. And, by all means, you can play it that way if you like. But - you can just drive around being pretty tame and have a great time. The media doesn't like to admit this.