How to prevent child from calling friends?

Even when the phone is locked to the maximum extent (bedtime on Google Family Link / Huawei parental controls), the child able to call friends. This is not what I want, especially during nights. I have spent a lot of time looking for a solution. This seems like something easy to implement (even locking the whole dialer app), however it looks like one would have to root the device to achieve this. I found one trail, *MMGuardian*, that claims to allow contacts white list, but this is paid software and I'm looking for a free solution. There is a Google topic about this, on which a lot of people voted. How come such a seemingly obvious functionality is not available?