Hello mrmarley,
Welcome to Apple Support Communities! It sounds like you need help with Screen Time. We completely understand why you'd reach out, and are happy to help.
Are both iPhones updated? This ensures that the devices aren't having a compatibility issue between the two operating systems.
Next, make sure that you have followed this article to set things up and are using a password, not easily guessed: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support
Other things to check for, is to ensure that there are no VPN's and that you've gone and disabled making changes the time zones: Go to Settings > Screen Time > Content & Privacy Restrictions > Location Services. Under System Services, ensure Setting Time Zone is toggled on. Tap “Don't Allow Changes” at the top of the Location Services page.
Please have a look and let us know if this helps!
Cheers!