It IS illegal in some parts of Europe (well, Germany at least) if done by the player themselves, and it could potentially cause an instant AO (or at least a nasty publicity incident) in modern higher end games, which is why Bethesda ended up shying from it in Fallout and Skyrim despite the fact Fallout 1 and 2 allowed it. In that case though it was probably for the best, people can mod that in on PC if they seriously want it for some reason, yet the fact it's not officially allowed means they can actually make the kids DO THINGS beyond being a nuisance like in the first two Fallout games.
As part of a game's story though? I don't think that's illegal anywhere (beyond normal violence laws) OR instant AO, and even if it deserves an AO they'd shy away purely because of how it's stigmatized by the industry and market. That's another problem altogether, and in this case they'd probably only want to "penalize" if it was actively a game about you killing anything. Unless something happens later in the story, in which case use the fucking spoiler tags.