Multiple reason why this will never happen:

1) Pedophillia from people (as much as we might not believe it's there, some people are screwed up mentally)
2) Child labor (by that I mean slaves)
3) Basically ingame pets you don't feed and neglect the whole time you're logged off

Why are we having this discussion instead of adding actual content to the game? Go play Sims if you're interested. Go play Skyrim and adopt kids. You can even set fire to them and they won't get mad!