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.
This. It's like slavery in Stellaris or genocide: it's so far removed from what entices empathy from these sorts of situations that it doesn't cause disgust (as long as you don't think too hard about it).