It sounds to me like you've already done a good job of identifying some of the reasons why the child might not tell the parent. Now the task is to reassure the child that their worries will be taken seriously.
As parents, our primary task is to keep our children safe, and there are certainly things we absolutely cannot tolerate. But if we are serious about this endeavour, we must also recognise that our chances of protecting them are best if they will trust us and confide in us. So there's a real balance that needs to be struck here.
On a different subject matter, a pre-metoo article on why men don't get to hear about sexual assault has this to say, which I think transfers quite well:
Men who hear these stories, I’ve found, tend to interrogate you to get to the truth of what happened, then, if they believe you, they want retaliation or revenge. Men want rules to be enforced and authorities called. Women want those things, too, but we understand the complicated mental calculations that are forced on us: If a man reaches under your skirt on an airplane, does that mean you should put your career, your ambitions, your livelihood in jeopardy just to watch him get some kind of slap on the wrist? Isn’t that ultimately giving this stranger more power over your life?
The reasons are different, and the assymetric guardian-protectee relationship between a parent and a child is lost in this comparison, but the lesson is still that if we really want to help someone, we need to truly listen to their story, and to accept that we should not dictate what the help should be. The reasons the child have for not telling might not be compelling or agreeable to us as parents, but they're real values to the child, and should be taken into thoughtful consideration.
I think the only real way to achieve this trust is to consistently take our children and their needs seriously, and respect their will. In a more acute situation, the parent may not have time for long term relationship building, and may have to resort to explicitly vowing that if the child has something to share, they will respect their feelings about it and not deal with it in a manner the child doesn't approve of.
The reasons you identify are real reasons that might well prevent a child from confiding in their parent about real ongoing harm. This needs to stop. Failing this, the parent is rendering themselves unable to protect their child. In that case, the best help for the child is to help them have other adults in their life that they can trust.