How do child-actors cope with horrific scenes? Do the directors usually try and avoid exposing the very young actors to the violence of such scenes by clever dubbing and editing?

The child actors are not exposed to horrific scenes as it is often believed by people.
Even before anything is filmed, the child actors are gathered together, and they are told what the film entails in case they don’t understand the script for some reason. The majority are film veterans, even at a young age, and so they are well-aware that the film is fake and that no harm will come to them on the set. In more modern films, where there’s plenty of CGI, the child actor will never see most disturbing scenes until AFTER the film is finished, or nearly so, as they have to be added later. Basically they respond to a green screen or another actor in a motion capture suit and there’s not much “scary” about that. Additionally, because of the limited hours that child actors can work in Hollywood, they film mostly in the daytime, and they are rarely working after sunset. In fact, if you look at most modern films, the scenes with child actors which are supposed to be set at night, are nearly always indoors, meaning that they were likely filmed on a soundstage and the lighting was adjusted to simulate external darkness. The few scenes which are filmed at night rarely have the child performer in them for long as again, strict work rules prevent child actors from being used in the same manner as adult actors.