Is it just because DF was developed on the fringe that it gets away with having infants and children that people use atom smashers on and have core game mechanics where monsters come snatch them?
Or is it something about the implementation that makes it drama-less to everyone?
As far as I'm aware anything that can happen to a Dwarf can happen to an infant or child.
I think its about intent. While the DF devs could spend more effort on keeping players from being maniacs, it still doesn't seem like the want the players to be terrible to children. In fact, some of the worst stuff (danger rooms as child abuse training zones, and mermaid bone farms), were specifically targeted in patches to no longer be 'good' strategies.
As opposed to a lot of other games, who's inclusion of 'punching children' seems like they're doing it to be 'edgy and subversive'. And the 'child snatching' thing is just classic fairy tale, and also would be a real danger if you're living in the wilderness around predator animals.
Plus afaik the child snatcher monsters just raise the dorf kids as members of their civs. Which, when you see what's going on in most dorf civs, basically makes them fantasy CPS.