At this time, not all app stores support the IAP model in Family Sharing. For more information on Family Sharing with iOS devices, please visit Apple's support page. For information on Google Play Family Library sharing visit Google's support page
If you want to try a workaround the best option would be to log in with the main account that made the purchases on the app store of each device. If you're on an Android device, the content should be restored automatically. For iOS devices please note that you need to access the shop, and tap on the "Restore purchases" button at the bottom.
You can read more about restoring purchases for each platform in the links below: