Seems like Briar might be good for messaging to young kids. I could give them an old phone without a sim card, and run Briar on it.
I'm not too keen on my kids being on the internet all day. In my case the opposite solution seems better, a dumbphone so I can reach them, but TikTok can't.
I am actually travelling 10 thousands kilometers away from them for a month, having an instant messaging app is a saver for both me and my daughters.
We are currently using deltachat for this with email accounts I set up for them. They don't even have access to these accounts outside of deltachat yet and they can't be reached by others like they would with whatsapp.
You'd have no idea how many accounts they have, who they're talking to, and about what. Perfect privacy!