I have no problems controlling access to sites, I can use open DNS, it's timed access for children to the internet I need to control. We had MAC rules set up for 9am-8pm schoolnights and 8am to 9pm on other days, and we can control the total amount of screentime using other methods. What we can't control using MAC filters is an automated shut off. It works for 24 hours and then their devices generate another randomised MAC address. Or they just power down and back up and it generates another MAC address. There is no way to stop this on some of their devices, and they can get round it on others.
It's as if no-one involved in tech understands or has the power/will to restrict kids screentime - or understands what parents actually want - probably becasue of the demographic of most geeks. And most companies actual business policy is to keep people glued to their screens with notifications, reccomendations and other teasers. Most parental controls seem just for show...
Have you looked at any options which are installed on the devices themselves to moderate content and control access times?
Past suggestions on here about restricting access at night have included simply having devices kept downstairs at night to charge so they are not in bedrooms overnight.
Having been involved with school IT in the past, I can reliably say that whatever tech measures are put in place, older kids will eventually find some kind of workarounds to defeat, or reduce, them and this can lull parents into a false sense of security thinking that the defensive measures are all in place when, in fact, the kids have defeated them some time ago!
In schools, the outcomes which were most successful were having the restrictive tech measures in place as a first line of defence alongside a shared understanding with the kids of what the rules were on using the tech and why they were in place. A bit more straightforward to impose that in a school but more a case of some delicate 'negotiations' in a family setting and particularly with older teens!