Blocking child's hotspot with Screen Time

My cellular provider cannot turn off hotspot functionality on my family's phones. I have several kids with phones which I control via Screen Time. Putting them on downtime at night is fantastic. I also have parental controls on the WiFi to our TV's, laptops, etc. and shut it down for bedtime. Recently, however, a couple of my kids have realized they can use their phone's hotspot to stream shows on the TV or provide WiFi to the laptops and tablets - even when their phone is on downtime! And let's just say...there are no parental controls in that situation. I simply get an e-mail from my provider telling me a child has used his entire hotspot data allowance...at 3 or 4 AM.

I've tried turning the hotspot off and blocking changes to Cellular settings through Screen Time. Some online instructions I found explained this should disallow turning the hotspot back on. It doesn't.

HELP!