Can’t create child account — stuck in verification loop

When I try to create a child account (Settings / Family / “Add Family Member” button / Create child account), I’m first asked to supply the child’s first and last name as well as birthday and to verify that I’m an adult by supplying the CVV code for my credit card. In doing the latter, I get a pop up dialog reading “Authorization required: According to European law, banks must authorize certain transactions” (translated from Swedish), whereby I’m asked to log in to my Apple account and then verify a payment of 0,00 SEK from my credit card. Having done this, I’m right back to the dialog and then it just repeats from that point.