I saw that question and your comment, and looking at the data it seems I was the one who handled the flag you raised. You asked for moderator intervention on that line, and I just moderator intervened on the question but didn't visibly do anything regarding that line, so I mishandled that flag and I apologise for that. In hindsight I should've left a comment responding to (and agreeing with) your comment, or something like that, so I apologise for leaving you in the dark.
I'm going to address this specific scenario and then your general question.
I agree with you that the stereotype is pretty bad. However, I also consider it to be an important piece of context for how this person is approaching this social situation and thinking about the object of the post. It's informative as to how we should frame our solutions to them — if we know they think this kid is a spoiled brat, we can remind them maybe they're not so don't do those things you'd do if they were.
My hope in that stereotype staying put is also that people, in answers, will challenge that stereotype just like you did in comments, and explain to them that being an only child doesn't mean someone will expect to get their way, so they shouldn't approach this child like that's the case.
Like, as a worse example, let's say someone was asking how to make a character for a girl in their group, and they said “she's a girl so she doesn't understand this stuff.” That is super offensive. Frankly it's terrible. And I could foresee answers spending a considerable number of paragraphs each telling this person off for their sexism, and telling them to treat this person with some damn respect, and telling them to just make their new player an ordinary character like they would for anyone else. If we remove that sentence, we lose context and don't get to respond to a major problem in the asker's perspective that needs addressing in the course of solving their problem. (Because they're creating most of the problem themselves.)
It's kind of a difficult situation because it's hurtful, but I feel that hurtfulness warrants being there to be challenged, corrected, and the person possessing the hurtful perspective being educated.
That said, the question also being closed means that isn't so likely to happen. We could revise it to be less matter-of-fact, but I'm torn between that & avoiding bumping the question.
If there's something you think we should do, or which should have been done differently (to what happened, or to what I'm describing) please let me know so I can better learn how to handle these situations.
Should you remove stuff you find offensive?
If it's clear-cut enough, like they're swearing or using slurs, please do edit it. If it's especially bad, edit and raise a custom moderator flag telling us there's an issue with a previous revision so the diamond moderators can be aware of what just happened and log it — we may keep an eye on the user and do further investigation.
If you're uncertain, please do request revision, and/or flag moderators for attention, as you've been doing. This flag wasn't handled the way it ought to have been. Part of the job of the moderators is working to make sure community members can feel safe from attack. Victims should not be left having to confront attacks upon them directly — they should be able to appeal for assistance, for another party to step in and assess things and help them out and (if necessary) back them up and support them. Your broader community (especially the moderators) are here to provide that assistance.
You can also check with chat, which has many even-handed considerate members active, or at least members who'll be happy to discuss your perspective and sanity-check it with you.
Sometimes we do have to walk into our own stereotypes. You might seek to get your way with a person complaining someone like you seeks to get their way. Diamond moderators delete material from users complaining the diamond moderators are engaging in censorship or similar. Don't let that stuff prevent you from doing what you know is right — would you do it if you were anyone else? If so, do it.