I've been looking for a good service/device to monitor and moderate Internet usage at home. As kids get older they get more ingenuous, and these days seems every single device has a browser and Internet connectivity.
Having gone through a few different services and not really liking any of them (mostly due to technical issues), and trying to deal with all the devices that aren't a PC but have a browser (Phones, tablets, Xbox, PS, Nintendo consoles, my freaking Kobo eRreader, etc etc), I decided to try this device:
https://meetcircle.com/
So far I'm liking it a lot. The setup was very easy and everything just works. All the devices on my network show up right away (and I get notifications when a new device it doesn't recognize comes online).
You can assign devices to a specific person (you create profiles for everyone). You can see a usage log, list of sites visited, etc. You can set a wakeup and bedtime so it can't be used outside that. You can set time limits, and set a "no device" time limit as well (i.e. all their devices won't do anything from 5-6pm for supper time). You can manually "pause" (i.e. disconnect) a specific device or everything. You've got granular control over what they have access to, and can blacklist specific sites.
It works via networking stuff in the background so you don't need any agents, it all just works, it's quite impressive actually.
There's even a Circle Go which I assume sets up a VPN for mobile devices so you can monitor/control even if the device is on mobile or a different wifi network.
It even has a battery backup so the kid can't just unplug it, and it'll notify you if something goes weird (i.e. getting the device off the network for example).
It's not cheap ($130), but so far I'd say it's worth the money if it keeps working as well as it has so far.