Hi there aidas227,
We see you need help with using Screen Time with your child's device and yours. One thing to check is to make sure their birthdate is correct associated with their Apple ID: Update the birth date associated with your Apple ID
If it shows them older than 18 years of age, that may be why. Also, make sure they still show in your Family Share. Go to Settings > Family to see if they still show listed.
Another thing to check is to make sure there are no OS updates to either your device or theirs.
Update your iPhone or iPad
If you still aren't able to set Screen Time for your child's device, reach out to Apple Support directly. You can reach them here: Get Support
Take care.
You can also find the support phone number for your region at this link: Contact Apple for support and service