Why is there no kid-killing in video games?

For example, in GTA games when you walk around in public, there's not a single kid around. It's like a world of only adults. So what's the deal? Are there hyper-sensitive watch groups out there? Will the game receive an "Adults" rating instead of "Mature" if there's kid-killing?

Let's be honest. Some kids really are lil' shits we'd like to wrangle.