Sounds like your iCloud account is signed in as the main user (top of main settings page) which is a common method of monitoring their communications (but has its downsides too) and you’ve configured Screen Time to lock your own account’s devices. Once you tap into Screen Time from the main Settings page, you’re looking at your own restrictions that will occur on all devices you’re the main account for. Lower down you’ll see your child’s account, once you tap that name it will open their iCloud account’s restrictions settings. It’s not very intuitive, but you can add a Screen Time widget to your Home Screen and select which family member to display to get there faster/easier.
As for the activity data, I experience that plus responsiveness to attempts to change settings pretty often and I have to force close the app - wait a few seconds - then go in again before it’ll work again.
If your account is the main account on their device, you’ll need to log yourself out and log them in, you won’t be able to see their iMessages without physical possession of their devices and the screen unlock passcode, or you can login to iCloud and ask them for the code that pops up on their device for security protection.
