Honestly, you can try as hard as you can, but they will find a way. Nothing is perfect, certainly not Screen Time. There are many threads with hundreds of people complaining about screen time settings being reverted. Half the time the activity doesn't even show up for me as a parent. It's quite honestly frustrating just how bad Apple butchered Screen Time. Be sure to restrict changing the date/time because just changing the time on your phone gets out of all Screentime time related limitations.

Now let's say, you have your children's devices on their own separate SSID on its own Vlan, with some DNS restrictions, oh wait they can just turn on their personal hotspot on their phone and connect their other devices to it because Screen Time has no way of disabling that feature, and AT&T has no ability to do this on their own (Verizon can do this however).

If they're old enough for a phone, they're old enough to figure out how to get around your blocks. Our 12 year old was informed of a Discord server (at school by other students) specifically for sharing proxy websites that get around website blocking on their devices. And the urls were all inconspicuous, like a website about minecraft skins has a subdomain just for hosting a web proxy.

You can do your best to block, but education around bad actors and what to look out for on the internet is far more valuable here imo.