There is a way to get around it, as it worked for me!đ
I turned on family sharing for my son. Now I have set up an Apple ID account for him too. His ipad was under my Apple ID account but I have since restored the ipad to factory settings, setting it up as his own now (you don't need to do this step, but I chose to do it so everything on his ipad is under his account now).
When I logged into the App store on his ipad, using his Apple ID account I can access the apps that I have purchased (I being the main account holder). However I had the same issue as you, that I couldn't family share the in-app purchases.đ
To get around this on his ipad, I logged off from the app store with his Apple ID account and then logged into it again using my Apple ID account. Therefore I could download the original apps that I had purchased for him + the in-app purchases. Then I logged out of the app store using my Apple ID account and logged in again using his Apple ID Account. Bingo now the full app is on his ipad!đ
Then I synced his ipad to the itunes library on my PC, so the apps are now in his library. It would be also wise to do a backup so you don't have to repeat this process again.....
Cheersđ