Adding storage to my childs apple ID fails

I have tried to upgrade the storage amount to 20 gigs on my childs phone but it fails every time and says not available try again later. She is in my family plan so my so she would be using my creditcard. Is this possible? I have it setup so her id asks for permission to purchase but not able to do this to buy storage for her account. How do I upgrade her storage. Not sure if this makes a difference but tomorrow she will be 13. Please advise her cloud is already at the limit of 5Gigs. Thank you, Lynn