(1) A separate, child-friendly version of the Delta Chat apk should be built. A parent would install this one for their child and lock down further installation on the device.
(2) The child would not be able to message any other person or join a group outside the allow list managed by the parents. The allow list may contain rules for both users and hostnames. Scanned contacts would first be forwarded to the parents for approval before adding.
(3) Any invite coming outside the allow list should be either ignored or forwarded for approval.
(4) Only WebXDC present on an allow list by the parents may be used.
(5) The disappearing messages timer should not be set to shorter than 1 week to maintain evidence in case parents come for daily check-up. Remote message removal and editing may also be restricted to allow auditing.
(6) Sending of self-uploaded or recorded media should be disallowed to combat blackmailing. The camera on the device of the child or at least its driver should ideally be removed and the parents would take photographs for the child on their own device.
(7) Sending of stickers and forwarding of approved media such as meme macros could still be allowed.
(8a) If server-side filtering is used instead of client-side filtering, a child should only be allowed to add an account hosted on a server managed by the parents. This may also be achieved at the router level if parents forced each device through their own VPN.
(8b) Server-side filters could be controlled by the parents using a bot.
(8c) The control bot may have a WebXDC interface.
(8d) This should ideally also be possible on shared hosts by allowing to register parent accounts and for parents to create dependent accounts. Registration would be completed over a different host and IP address so it would not be present on the network level allow list on the device of the child.
Attempts to control children like this tend to fail badly. The kids are more likely to learn circumvention skills that might be better learned when they have more maturity. They will circumvent the controls if they want to, and often even if they don’t want to, just to assert their independence.
Educating kids and helping them make good choices always works better than coercing them. In-person supervision until you are sure they understand is appropriate. If they are too young to understand how to do something then they are too young to do it.
Adults, on the other hand, have less free time and ability to learn than kids do. It is also easier to scare them. The suggested controls would be much more effective if used against, say, adult employees. Or an authoritarian government could use them to make a locked-down spyware app.
Controls placed on children can be placed on anyone. Deltachat should not assist censorship and authoritarian control.
A tool is just an option - nobody is forcing a parent to use it. A parent is free to install the full unrestricted Delta Chat for their child depending on age and maturity.
How to handle each kid can vary wildly even within the same family. What works for one may not be the best way for another.
No such FOSS alternative exists so far, so a responsible parent has no choice but to use a privacy intrusive proprietary service.
You are ignoring the rights of the child and the moral certainty that the tool will be used maliciously. A tool is not just an option if it is being used to coerce users. Somebody is forcing the kid to use it.
I think a responsible parent should help in person until the child can do a thing independently. A parent’s job is to raise an independent person who controls themself.
Deltachat does not make money from scams and blackmail and fraud, unlike certain popular proprietary platforms, nor does it gain by being addictive. I think it is safer.