In-app purchases not working with Family Sharing

downloaded an app and purchased to unlock things in the app for my child. Now my child has their own device but I cannot seem to transfer the purchases. He can download the app but everything I already bought with my Apple ID is locked. His Apple ID is added to my family and app/ purchase sharing is turned on so I’m not sure why the purchases in the app are not transferred. (Specifically talking about the Lego duplo apps) .

[Re-Titled by Moderator]