That’s a tuff ask, and it’s been awhile since I looked.
My kids phone is Android, computer is Windows, and I couldn’t get away without Googles Family link.
The PC was easier. I use Pihole to primarily block ads, but it can also log DNS requests from my kiddos PC, and I use my switch to kill his internet. It’s a desktop and doesn’t have wifi. So it’s just a Ethernet cable to his room. Kill the port or unplug the cable for dramatic effect. Any OS he’s on he’ll have restrictions and monitoring as long as he doesn’t spin up a VPN or learn how to change the DNS host. But much to my dismay I don’t think he’ll ever love Linux, or networking.
The phone was harder. Android gives up a random MAC address so blocking him from wifi was difficult, and it didn’t matter anyway since he had cell service. Eventually I came to realize that if he’s in the Google ecosystem, I have to play with Google if I want parental controls on his phone. It sucked. But, again much to my dismay, he doesn’t seem to think a big company knowing literally everything about him matters all that much so… Whatever. Maybe I’m the one that’s crazy.