She's using a Samsung android phone with Family Link, it's meh but does it's job. I'm looking into limiting screen time more than content restrictions, and having the total screen time across all devices in a centralised service is very much what I'm looking for.
Seems like this topic got lots of attention over night, I really appreciate it!
It looks like there's really no solution for what I'm looking for, even if I'd move to windows I don't think I'd get what I want. Apparently only Apple has that but I'm not sure, never owned an apple device.
Seems like this would be a cool project to work on, cross platform cross device parental control with Linux as a first class citizen.