Unable to install app on childs iPhone

Hi

Our 12 year old son has an iPhone with his own AppleID. We have setup Family Sharing and have the "Ask to purchase" app setting which works fine.

We cant however install a "paid for" app. Anytime we try to install this on his phone, after entering his AppleID password a message appears saying "Verification required" and then takes us to the payment methods menu. We have added £10 to his Apple account, and when you go into manage payment methods o his phone it shows his account with a balance of £10 however when we try to make this app purchase it continually asks to add a payment method (after clicking continue on the Verification required message).

I have tried turning off "Ask to purchase" but it makes no difference.

Again, this appears only to be an issue for apps that require payment, free apps install fine without the message.

I can only think this is something to do with it being a paid app and the fact its a child apple ID (under 13) attempting to make the purchase but I cant see any Apple document that discusses this.

Does anyone have any idea on how to fix this?

Thanks