Do you want honesty? No chance. Anything you can do that limits their access to your home network they can get round by talking to their techy friends; and will also cause you serious inconvenience. You may not want to hear this but the best way is to lock away their devices at night - unless you can negotiate a deal with them on self-limiting their access.
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So there are a couple of ways you can go about this.
The first will require quite a bit of tech know-how and involves sticking another device in your home network. - Endian Firewall would allow you to keep tabs on their usage, block certain sites, set time profiles etc, however it does take some setting up and management.
The second option is something like K9 Web Filter. Now it has been quite some time since I used this, instead I went down the Endian firewall route as I have extensive experience on networks and infrastructure, but I think K9 may suit your purpose. Be warned though... teenagers are a different breed!... expect Kevin and Perry except on a much worse level!!
I think the Hub3 also has time profiles set on it, however that doesn't help when they are out of the house, whereas the K9 app does (or at least did!).
Thanks for using the forums to get help with limiting your child's internet usage, I am afraid that we can't do anything on our side to stop them from accessing the internet at specific times, however we do have child safe guards and other internet security features which I think may help. Here is a link to some helpful options for this. I hope this helps, but if not, you also have the community members to help you with options outside of Virgin.