Did you elect to share the storage and did they elect to use it? See the information in Share iCloud+ with your family - Apple Support including (but not limited to) the following:
If a family member is already paying for their own iCloud+ 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.