Family Sharing

Don't enable purchase sharing. From the article below.

 If you turn on purchase sharing, you'll be asked to confirm that you agree to pay for purchases initiated by the family members you invite and that you have a valid payment method on file.

Family Sharing - Set up.  *

No. Purchase sharing, as its name implies shares purchases and makes you be charged for the purchases they make.

If you turn it off, you neither share you purchases nor are the purchases charged to you.

You cannot have one without the other.