Preventing device from accessing open WiFi networks
This is not possible unless you restrict access to the device's Wifi itself, which is not possible on current most popular OS's(not without unofficial modification atleast) Android, iOS and Windows Phone.

There are apps that can control accessibility of porn websites(Apple has its own) on the device itself(thus it will deny requests to porn websites from the router), but if your kids really want to, there's always a way around the filter.

Maybe it's time to think that censorship isn't a modern solution to these kind of things anymore.