Since when was GTA marketed towards children? The Grand Theft Auto franchise, ever since GTA III, has recieved mature ratings, meaning that anybody under 17 shouldn't play them at least without adult supervision. If you have a problem with the adult content, then don't play it. Just because a game includes adult content that you don't find suitable doesn't mean that the quality of the game is bad.
And before you say "well all video games (should be) are made for children to enjoy". The average gamer is 31 years old. Game developers aren't going to make content that is oriented for a 10 year old if the average gamer is a full grown adult.
Rockstar Games or any game developer for that matter has no control over what parents let their children play alone. So if you see lots of children on GTA, blame it on their negligent parents and not the game itself.