1. Don't buy your kids a cell phone until they are ready to drive.

2. Force all home network traffic through a DNS service you manage, for example a PiHole.

3. Back up the PiHole with a commercial DNS point that also filters like CloudFlare Family DNS (its free).

4. Every few months inspect your children's devices for their internet activity, VPNs, and other things.

I suspect there will be many excuses about how children cannot possibly live life without a cellphone or about how draconian these rules are. The reality is you are either being an active parent or you just don't really care.