I have family sharing set up on my son's iPad and when he wants to purchase Minecoins, worlds or skins he has to 'Ask Permission' that sends a message to my iPhone and I authorise the payment through my PayPal account. 

In Marketplace we select what he wants to purchase and usually click on the button showing the amount. To complete the purchase we do the following:

1. I enter the password for his iOS account to verify the purchase.
2. It says he has to 'Ask Permission' we click OK
3. I authorise the request on my iPhone (not running Minecraft).
4. The authorisation of the purchase shows on the iPad (running Minecraft Marketplace)
5. Marketplace does not complete the purchase and just sitting saying 'It won't take long...

Normally to complete the purchase you have to restart the app, go back into Marketplace select what you've just tried to purchase, choose to purchase it again and it detects the payment has been made.

Marketplace should detect the payment has been authorised and complete the purchase without having to close and reopen the application and repurchase what you have just paid for.