Ask to Buy blocking WhatsApp download on child's phone - Unable to ask permission

I have started Family Sharing with my 12 year old sister in order to have control over her content, as requested by my father. I was appointed as Parent/Guardian in the family sharing group. After we set up all the limit we deemed appropriate she started downloading the apps that we approved through ask to buy. Everything was working and downloading perfectly (including other apps rated 13+) and I was receiving all requests for downloads until we tried to download WhatsApp, which is the communication tool we use as a family. When she asked for permission to download WhatsApp we got the following error message "Unable to Ask Permission - You can't ask permission because you have signed in with iCloud and iTunes accounts that are not associated with each other". Which was strange since she only has one Apple ID.

Steps taken to try to fix this issue:

1. Checked Apple IDs on iPhone, Macbook and App store. All the same.
2. Checked Media & Purchases
3. Reset Ask to buy.
4. Reset Screentime.
5. Restarted both phones.

Currently have the app store age setting to 9+. A workaround was to set it to 13+, then she was able to download WhatsApp but that messed up Ask to Buy (no longer getting requests) and Screentime became very delayed.

Any help appreciated.