Considering that this minor character is torturing the cover character, that information is actually critical. As a reader, I would feel cheated if you as a writer glossed the details of an event that was important enough to use as the key promotional item (cover) for your story. That cover is a promise for your readers. If I can't compel a reader to care about or love to hate your characters once that person is reading the story, then why would a reader bother to reading your story unless there's a torture fetish involved? If satisfying a mindless torture fetish is your goal, then you have achieved it marvelously. I have a feeling that's not what you are after.
If you want the reader to care about the little boy's fate or to hate the bully on an emotional level, you need to start caring about the details. When you don't care about your writing, it shows. Sometimes minor characters are huge assets that suck your readers into fictional universes (Bobba Fett, for example.) Treat your creations and your readers with enough respect by paying attention to the small things. For example, in A Broken Winter
(extreme violence warning), there's a torture scene conducted by very minor characters that reminded a lot of yours, even if the motivations were vastly different. In the very first page, you know EXACTLY why the child is being tortured and what is at stake. In your story, I am not sure why the bully is so brutal about the amulet. Does he know the value? Does he just hate the miner children? Why is he evil?
As a Puerto Rican who has been to Central America, (Guatemala and Southern Mexico, to be precise) I've observed that the most common self-defense tools tend to be insults, knives or fists. Maybe it's different elsewhere. But in my experience, unless the kid deals with cattle or sheep, the whip seemed to come out of nowhere. Also, if the kid is a hoodlum, the chances he'll have a gun, are higher than that of the general population. Maybe it would help if you assumed that not all of your readers are familiar with life in the specific area of Central America that you are dealing with (especially since the story is written in English) and would appreciate some minor tidbits (even within the story) introducing them to cultural nuance.
I read (and write) a LOT of fiction inspired by horrifying events in history, whether it's sci-fi or historical fiction. I am your target market. I am passing these observations along to you because your requested feedback. Take or leave what I mention. But realize how your work is making me feel as a heavy consumer of that type of work.