I think the issue is the label "18+". It's clearly arbitrary. In many countries women's breasts have little to no stigma attached to them and are freely shown in media allowed for children. Even more so for sexual innuendos and themes. On the other hand, things I'm sure you would consider "cartoon violence" are outright banned or just not considered children's content in these same places. (In America we normalize showing guns to children, but in places with more gun control a gun on TV can be shocking).

So when you (and not just you, many people have this same sentiment) say

Kids (talking 10 and under) shouldn't be subjected to media or adjacent media that deals with 18+ adult themes. Period.

You're appealing to a very very narrow definition of 18+ defined by your upbringing, time period, and region. To the poster before yous point, it's pretty clear that kids are adaptable and are much less negatively affected by "adult themes" than we fear monger about evidenced by my earlier point that kids around the world are raised with wildly different standards as to what's "18+" and they are all able to grow up into functioning members of society.

Now from what I gather about Hazbin Hotel it's a pretty adult show, I've never seen a frame of it, but after looking it up it's rated 16+? And people online are saying it could be fine for 13+ with adult supervision?? So how nasty can a panel get? (Edit: sorry I thought it was an "adult" adult show. This is just a YA show, seems like a lot of todo about nothing. Maybe the children were loud and it got annoying). I really don't know. Maybe the show is extra gross but I'm not defending this particular instance, just the topic of this one thread.