I have had Family Sharing set up since it was introduced. Now that iOS 12 offers Screen Time I want to configure it for my family members, but only one of the three people in my family, not including me, appears under the Family section when I go to Settings -> Screen Time. Same if I go to my Apple ID - > Family Sharing -> Screen Time (under Shared Features).
Here is the support article on the topic: https://support.apple.com/en-us/HT208982
The part that stands out to me is this:
"To use Screen Time with Family Sharing, you need to be the family organizer or parent/guardian in your family group, on iOS 12. Your child must be under age 18, in your family group with their own Apple ID, and on iOS 12."
They likely don't meet one of these criteria.