Why are My Family Purchases are Being Charge to Me

No, iCloud+ (storage space) is one of the shared subscriptions.

Apple notes in Change Purchase Sharing preferences in Family Sharing on Mac that: When you stop sharing your purchases, your family members lose access to all shared iTunes Store, Apple Books and App Store purchases, and aren’t able to make new shared purchases. All these are bought media content which then becomes playable only on the Apple ID account which has purchased it because it is no longer shared.

And in How to share purchases with your family Apple explains: When purchase sharing is turned off, you can continue to share subscriptions, such as Apple TV+ and Apple Arcade. 

The last link above takes your to How to share purchases with your family in which Apple says:

Who is charged for the subscription?

If Purchase Sharing is turned on, the family organiser will pay for everyone's subscriptions.

If you're the family organiser and you don't want to pay for family members' purchases and subscriptions, you can turn off Purchase Sharing. When Purchase Sharing is turned off, each family member must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions but they can't share individual purchases, such as apps or books

Your subscription to iCloud+ is safe and will continue to be shared with either setting.

Read How to share purchases with your family

When you turn on purchase sharing, everyone in your family gets access to apps, music, films, TV programmes and books that family members buy. The family organiser will be billed for family members' purchases.

If you turn off Purchase Sharing everyone will be required to use their own payment method - but they won't be able to share family purchases.