Family Sharing (Change Region)

Our family has just moved from the U.K. to the U.S. and so we need to change the region on all our devices to access local content (bank apps for example).

I have tried changing region but it says I either have subscriptions or I have to disband our Family Sharing.

I have no subscriptions, so I believe family sharing is preventing me from changing region. On the sharing, I am the ‘organiser’ and I have two children. I can remove one child who is over 13, but not the one who is under 13. Until I remove him, I cannot disband the group.

So my wife set up a new family group and sent him an invitation to join, but the message said he had to leave the other group first, yet there was no instruction how to do this.

How can I move him to the new group created by my wife, and therefore disband family sharing?