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.
Edit: reading the rest of your comment I see that this doesn't exactly meet your 'voice and video' specifications. Apologies.
10 or so years ago when Minecraft was coming out of beta, I was a precocious enough child to run a server on some spare hardware lying around the house. (Realms wasn't a thing until like 2014)
Good times were had between my friends and I, but that all changed after I started becoming intoxicated by the power. Whoops.
Give it a try, but I advise against giving a literal child the ability to ban people. I would know.
The kids usually start a voice FaceTime group chat and then start up Minecraft, which keeps the voice chat going in the background. It works great. (If they are playing with someone that doesn’t use FaceTime: I’m pretty sure that the Facebook Messenger for Kids app also works)
I’ll also put in a plug for the Apple Watch cellular - we have them for the kids using the family setup. The kids then have a phone without having a phone, they have the blue iMessage bubble in chat, etc. But no camera, no bad social apps, etc. And you can have them delegate management of their address book to your phone and then set the watch to ignore everything except contacts in the address book. A side benefit is that each watch is $10 a month instead of the $30-50 per month a real phone costs.
MC is good because even if they’re not into playing the game they like just text-chatting while they walk around looking at stuff or exploring premade worlds from the free section of the store.
I'm tempted to say this would be less addictive than a smart phone, but thinking back to being a kid, my sister was almost always on the phone talking to her friends, so maybe not. But it at least restricts you to talking to people you already know and doesn't enable meeting arbitrary strangers.
It's made by a gross company, but they have ridiculously tight access controls.
The main challenge I ran into was, of course, convincing folks to use it (instead of the "easier" forms of communication that they already had....).
If I rolled my own it would be a little like Slack, probably a single channel with threads and like a 30-day history, to keep it pretty ephemeral. The logs would be in an admin feed I would keep an eye on.
I’m not sure how long you could keep that level of control and keep other technology at bay, but it’s worth it to avoid premature connection the matrix.
Another option would be nextcloud talk (also should be self hosted)
Even if you don't like FB that much, you should look at what they are doing in messenger kids
> FB
Thumbs down.
