neither one works flawlessly in isolation. You can explain to a child why you don't want them doing something until you're blue in the face. But in all likelihood they're going to want to do it more. The backup is then to block the app on their phone

Sure, they can still use their friends' phones, but you can only protect a child so much. You've made a good start by explaining the issue and banning the app, but you can't track them all day to make sure they're not doing it elsewhere.

Instead of focusing their attention by telling them how bad it is, give them something else to do and introduce you them to other children raised with similar values

You can do that all you like, you can give them all the hobbies they could ever need to be busy. But eventually one of their friends is going to share the app with them when they're playing together

> and then they will just use the phone of their friend with no such policing in place or use a publicly available client with no such controls in place

That means, that they wont have that content pushed on them for hours either before sleeping or while you work and they are in home due to lockdown.

They will have access to it only while they are with that friend.

I deploy a novel virus that makes it impossible for them to get near a friend's phone.