There does not appear to be a way to disable the option to provide one more minute. That is confirmed at the two threads below.
"Child accounts can click One More Minute once, or click Ask For More Time to send their request to the parent account for approval."
As it turns out, this option cannot be disabled in Settings.
An Aside: Restrictions on vanilla iOS are not very robust. If more restriction and control options are desired, a configuration profile may be created and applied to a "supervised" device. Even then, such a simple thing as enforcing DNS on cell signal is not possible unless you create an always on VPN. App whitelists or blacklists however are possible along with other useful restriction options. You can use the Apple Configurator 2 app on Mac or visit the documentation to see what is possible: https://developer.apple.com/business/documentation/Configuration-Profile-Reference.pdf
Note however that using app whitelist only mode might cause issues with apps not showing up on a connected Apple Watch. I have not been able to find a solution to this.
Aside Aside: I have decided to move to Android as it gives much more control to the user in terms of allowing them to restrict or not restrict what they want. I have taken the iOS restriction options to their limits and they are still found wanting.