My child can't make in app purchases

Hello,

I have a problem with in app purchases for my child on his account. So, I've created him a new Apple ID, he is 9 years old, he is in my family and I'm his guardian. I have a credit card in my account and in Apple pay and it is working.

I have purchase sharing enabled. When I turn on Ask to buy for him I get the message every time he wants to buy the app in app store. So that is working too.

The problem comes when he tries to make an in app purchase; for example in Brawl Stars or Clash Royale. It doesn't ask me anything but it simply doesn't allow him to buy anything. It doesn't even come to the payment part. It is just cancelled before even starting; "purchase is not available", "you have not been charged" or something similar appears on his phone.

I can't insert my card / Revolut to his account because he is 9 years old so that is not an option; Apple Cash is available only in USA. So I don't know what to do...

He has his Apple ID, he is part of the family, sharing the same subscriptions for Music and Arcade, so everything was going well except the fact that he can't make in app purchases in games is a very bad thing.

If anyone has a solution - I (and especially him) would be very grateful.

Thank you very much :-)