The problem is that the vtuber community is much more tight-knit space and it's a lot easier to fall further and further into it than mainstream internet content which is more spaced out with less crossover. We refer to it as a rabbit hole for a reason.
There's a handful of genuinely wholesome safe-for-work streamers who, in a vacuum, are perfectly harmless for kids to watch; but it's much harder to make that recommendation when the viewers can stroll over to Twitter to find fanart of them getting piped, and eventually get clips in the sidebar of other vtubers who do lewd ASMR and have Discord servers full of horny fans who likely aren't the most morally upstanding individuals.
It's not realistic to filter out kids completely, but given how prominent sexual content is in the wider community it just doesn't feel responsible to invite them in.