I don't know. There is a lot of hate for six-year olds, mainly because there's that stereotype of kid gamers being annoying on servers and scream "I CALL HACKSSSS" in chat or something. (Like I said, its a stereotype. Not every kid is annoying.) Maybe this is revenge? Some people are just mean and like hearing others get upset. It doesn't really make sense to me so I stick with YouTubers that do legit mod reviews and games.
Yeah, I get that little kids can often be annoying online. However, these YouTubers are going out of their way to "befriend" children on XBox Live(usually) and then proceed to permanently mess their world up, and when the child gets angry(obviously), they act flabbergasted. I mean, a lot of adults would probably be quite upset if someone came and destroyed something they've put hours into, why is it so strange that 8 year olds get angry? As you said, it's more that these people themselves obviously enjoy harassing others and then making a buck off of it.
Mojang does not have any rules about griefing or trolling on servers. A big part of their philosophy is that servers should run themselves. Xbox on the other hand has more specific rules about that kind of thing but even then destroying builds in Minecraft is kind of "iffy" because it is a legitimate part of the game, and more often than not it would probably have to be reported within the Xbox's report system for it to really get to people at Microsoft who do that kind of work. Trying to ban users over YT videos which could be falsified and staged is much more risky than dealing with reported messages and players.
Depending on the content of the videos, the best bet would be to report it to YouTube directly, as they are more likely to remove videos of people harassing others than anyone at a different company seeing it and taking action.
I had the impression that "trolling" was against Minecraft's ToS due to the ZexyZek scandal some time ago, but I guess it might not be anymore(or it wasn't in the first place either and I completely misunderstood some things).
Reporting to YouTube seems like an interesting idea, but I personally doubt it would work - I assume that a bunch of people have already reported said content, and nothing obviously came of it. I might give it a shot, though.
They sometimes show up in my feed too, in fact one did just a few hours ago(and that was what got me to make this thread).