I would go with Deltachat. There is no public directory of anything, so it's impossible to know who is/isn't there or randomly contact anyone. The only ways to start a conversation/follow someone are:
- scan qr codes, so you need physical presence and it's easy to see when there is no consent
- clicking on a random link
It i safe to tell your kids to not click on random links, so it's the safest way for kids to not randomly come into contact of people