So, why isn't this a thing yet?

[MisterLucie]

Maybe SE will cave in like they did for Male miqote and the other genders being added...We just have to keep at it for 9+ years XD (Personally I think that helped the game a lot )

But lets ask ourselves this: Would adding playable kids really hurt the game much? (Or at least teenagers, so the Tumblr SJW's won't throw a tizzy)

and If so, how and why?

I think a better question would be whether or not the pros outweigh the cons.

Cons: SE is inevitably hurled into a blazing firestorm of controversy for allowing children to be maimed, mauled, and sexualized.

Pros: Some people get to play as a child, for whatever reason.

Seems pretty obvious.