Well, i'd think that nerfing the world would seem less likely, but honestly, did you nerf the fairy tales? In Red Riding Hood, the Wolf gets killed, i can't imagine how many cartoons nowadays deal with violence...
Letting your 4 year old kid play GTA or some subversive morale-oriented games would really be kind of stupid, but nerfing the awesomeness of Dwarf Fortress is just a little bit overprotective...
This "violent games make violent people" bull is far from true, violent ppl come from violent homes, you just have to keep your kid from addiction to games, if he knows the difference between in-game fantasy violence and real violence, you'll be fine. Just keep yourself from violence and teach him to avoid it.
I'm an advocate for the Games, f'ed up ppl are susceptible to transferring violence from games to real life, but they were already prone to it and could pretty much learn from violent movies or ordinary daily news.
Unless you're Vegan and proud, don't nerf anything, will make the game boring and he'll hate it nonetheless.
Learn with your dwarfy fellow, hammering ppl on the head builds character