but in a game in wich you already can do sick stuff to adult people, why not?
I voted both, btw
I don't think you fully understand the way the media works.
On the OP's question, I think kids would be fine, as long as they only show up in cutscenes, or in other situations where you CAN'T hurt them. I'd normally say 'Sure.' to animals, as they've already been kill-able in many games, but since this IS GTA, and the only animals you'd see would be domestic ones, I think not. Media would eat it up and spit it back at R*. I think we all know it.
But then again, as soon as you put in these models, some modder will find a way to put them back in harm's way.