[KingMachiavelli]

Slack and Discord both have options or default to preventing external messages. Discord is probably the easiest since you can just make yourself a moderator which gives you a lot of power. If you need to prevent curious kids from joining or accessing other servers/groups then that's a lot harder since eventually they will learn to use Discord via the web browser, etc.

You could run a Mumble instance for voice and text communication. [1] And just give everyone a Jitsi video conferencing link (needs to be kept private/secret or rotate it often). [2] The downside to this option is that every kid/parent needs configure this.

[1] https://www.mumble.com/ [2] https://meet.jit.si/