We’re sorry about any confusion!
Currently, Apple and Google don't support family sharing for in-app purchased products.
Family sharing allows family members to share eligible content. In-app purchases, such as unlocking features of a free app like Drops, are extra content, and they, unfortunately, are not suitable for family sharing.
You can find more information about this by Apple here:
What types of content can I share with my family using purchase sharing?https://support.apple.com/en-gb/HT203046
and by Google here:
See if content is eligible to be added to Family Library
We constantly work hard on improving, including a solution to this. We can’t give you a time estimate when a family option will be introduced, but please stay tuned!