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I have a feeling there's nothing you can do about access via a cellphone (without denying the cellphone any access, as in not being able to make or receive voice calls).
When it comes to the computers though, one hardware solution would be to use the access restriction controls in your router. Another would be to utilize the hosts file on Windows machines.
You're correct, you can't be there at all times. Parental control software is just one more thing you'll have to set up and monitor. And kids figure out a way around it, QUICK.
When you're trying to teach kids - especially teens, who we're training to set free in the world - forcing their hand only makes them work harder against you.
We tried all this, including software, and none of it worked. Had a problem with a teen and video, killing our satellite bandwidth (which puts us in lockdown for 24 hours if exceeded.)
Warning one: We get another FAP, we unplug you for a month. Your choice.
Warning two: Next time, we cut the cable.
Warning three: there is no warning three. You have a job, get your own satellite account. You're done.
May seem a bit harsh, but as our kids get older, we aren't helping them by being a helicopter parent. Eventually, consequences are the only places real lessons are learned.
For children below that age I would suggest a positve list of websites they are allowed to visit in the access restrictions of your router while blocking all other websites.