I use Matrix daily but I would hesitate to recommend giving it to children unless you’re able to set it up in such a way that they only have access to the rooms you configure–I don’t think any Matrix apps have parental control settings but I’ve never checked.

Matrix has kind of miserable moderation options. Mods can’t do basic things like disabling attachments, which makes it prone to NSFL/NSFW image spam. The official automod bot, Mjolnir, can mitigate this somewhat, but can be pretty easily gamed. e.g. It tries to prevent image spam by deleting messages that have an image if they’re the first message a user sends in the channel–so spammers just send a message to highlight everybody first, then the images.

When there’s nothing that needs moderating, Matrix is great. However, it is severely lacking in ways to handle abuse. If you’re in a community that’s being targeted by bad actors (like Jared Leto), you basically just have to deal with it; there’s very little you can do proactively.