It just doesn't work very well.
The first problem - and it isn't limited to screen time but other apps of the ilk- is it doesn't switch the machine off at the set time. It will only block them if they switch off and then try to go back in again. If they leave the app open it will allow them to carry on forever.
Secondly, it only works with certain apps. Somethings don't get picked up at all. From memory Youtube app isn't picked up as screen time (but it's been a while so I may have mis-remembered). Also if they use Chrome and not Safari I think.
Thirdly, it seems not to coordinate well between devices so if they are using a phone and an ipad for instance it just doesn't do the addition.
I gave up on it as it was never correct- not even for me and I wasn't trying to beat it just see how much I was using my devices.
The only thing I got to work was a modem with good time controls available in the interface. Then you can block content, time and device so you can block social media and streaming to their device after 7pm but allow them to do their homework for instance.
You can also block adult content and run anti-virus before it gets to their machines. It's a bit of an investment but it's by far and away the best solution
Another trick that is useful in some circumstances is to use a third party DNS service with controls. Open DNS for instance will allow you to block sites and content and it's very unlikely they'll ever figure out how.
If they have a phone with a data plan though then they can always use their data and it's pretty easy to set up a hotspot too, so nothing is a total solution.
A combination of things is the best way and an alarm going off at a set time when they have to hand in all devices.
Afterthought: you may already have time controls in your modem so it's worth checking out. You'll need to find how to get into the interface (which is usually 192.168.0.1) and you'll need the log in and password although they are often 'user' and 'password' if you've never changed them. Otherwise they are likely on a label on the bottom