I'm not sure why you dismiss that one. Whitelisting apps works - that's the whole idea behind kiosk devices. There are guides available, but basically you enable only the allowed apps, uninstall webview, lock the account.
Yes, it's not trivial and not everyone will want to do it. (It's available though https://support.google.com/families/answer/7103340?hl=en#zip...) That's exactly why I wrote the last paragraph. Full access was one extreme. The ban is another extreme. (And is not exactly easy to implement either)
Once enough places overreact with the complete bans, the lockdown mode will get better documented, more popular, known by schools, finally (hopefully) easily available from the installation screen.
> because is not applicable today by parents
That's ok. This problem will still exist in a couple of years and this will be a bit easier to do each month. It was actually impossible to implement a few years ago, but we made progress.