Can't update trusted phone number for child's iPhone setup

I have bought my son an iphone and as part of setting it up it needed to send me a text, however the trusted number is an old number, I have checked my account and my new number is listed everywhere with no sign of the old number anywhere. How do I change the trusted phone number on Family sharing to the correct one?

[Re-Titled by Moderator]

Original Title: Setting up kids iphone trusted number