Unable to create a child account: "Your account cannot be created at this time"

I had the same as what happened in this thread: Unable to create a Child Family Account - Apple Community

I tried to create a child account (region: Canada) for my kid who is 9 years old, from a MacBook running MacOS Ventura 13.7.1 (most up-to-date version compatible with my model).

I go through the payment card verification, enter my kid information, accept the terms and conditions, and it ends up with the unhelpful message "There was a problem creating your new Apple ID. Your account cannot be created at this time."

I made attempts both with an icloud email address, and my kid's email address.

I tried disconnecting then reconnecting my Apple Account. I tried rebooting.

I have the feeling I tried about everything proposed by Apple to solve this, except trying from another device. I don't have an iPhone or an iPad, so I can't create an account on something else than this MacBook.

Honestly, it would be convenient if there was a way to create a child account from the https://account.apple.com website.