Hello Eskimo259,
Thank you for using Apple Support Communities. If you've properly set up Screen Time and you're unable to manage your child's device, we can recommend some steps that should be able to help you. The first thing you're going to want to do is to confirm all devices, yours and your child's are fully updated on their iOS version.
Keeping your device fully can help with performance and also help with sorting out various issues. Just in case you do have pending updates on both devices, you're going to want to follow these steps here to get updated:
Update your iPhone or iPad - Apple Support
1.) Back up your device using iCloud or your computer.
2.) Plug your device into power and connect to the internet with Wi-Fi.
3.) Go to Settings > General, then tap Software Update.
4.) If you see more than one software update option available, choose the one that you want to install.
5.) Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
Feel free to keep us posted. Thank you for using Apple Support Communities. Cheers.