re: "I've read the support articles that state that during the sharing process, I have the option to choose how much storage to share with each family member."
What support article is that?
Unless there has been a very recent change that I'm not aware of and the article Share iCloud+ with your family - Apple Support was'nt updated, you don't have any option to choose how much is shared with each family member. Any family member can use any or all of the available shared storage plan.
If the family member only had the free 5GB plan when you shared the storage, they should have automatically been added to the family plan storage. If they had an upgraded plan, they have the option to stay with their own plan or to switch to the shared plan. See the information in Share iCloud+ with your family - Apple Support including the following:
When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.
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.
If you shared the storage as described in Share iCloud+ with your family - Apple Support and they don't have access to the shared storage, contact Apple Support. See: Contact - Official Apple Support
