It takes a combination of a few technologies to be effective imo
- Screentime limits: have a "bedtime" set for each device. Some applications are excluded from this purely from an emergency communications perspective.
- Family-friendly DNS services enforced: Use a trusted DNS service that can block access to anything adult themed. It needs to be customizable though so that additions and exclusions can be made. Lock down devices so that DNS cant be changed without proper permissions.
- pre-approval of apps: be the judge of which apps are allowed on their devices. Do some research before installing anything, and keep up with those apps so that you can make changes if the apps start doing different things later.
- kids have their own wifi network: this makes management much easier. You can set the hours and protocols that are allowed on their network. Makes it easy to change DNS servers if needed.
None of this is easy to do, and honestly, it just seems inhumane for parents to have to go through this. If they don't do it though, the children can have much more exposure to various content (some which won't be ideal).