Did you follow all of the steps here:
Share an iCloud storage plan with your family - Apple Support
Basically, the family organizer must have the 200GB or 2TB iCloud storage plan, iOS 11, and choose to share it with family members by sending them invitations and they must then opt to join. If a family member is already paying for their own iCloud storage plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.
If for some reason, you bought the 200GB or 2TB plan and it is not showing on your device, try signing out of iCloud, then restart or force restart it, and then sign back into iCloud.