Adding debit card to my son's Wallet - Not allowed

I'm trying to add my son's debit card to Wallet on his own phone (with a child account, he's 7 yo). When clicking Add Card i Wallet I get a message saying "Cannot add card. Payment cards are not supported for this account". Why is this and what do I do to approve him having his own debit card (connected to his own bank account) in Apple Pay /Wallet?

Using iOS 16.3.1

Thanks,

Joakim