Change Apple ID email address for a Family Sharing member

Hi,

I have tried to change the email address on my daughter's Apple ID, she is part of my family sharing set up. As she is 10 years old, it says 'Parental consent is required to make this change'

"An email request will be sent to your family organizer, who can provide parental consent to make this change."

So I type in the new email when it asks me for it (which is an email listed in her 'reachable at' email list) and it asks for her password, then it logs me out of appleid.apple.com. No emails get sent to me, her old email or her new email. Her current appleid email is an iCloud address, and I want to change it to a Gmail address. I think this is the problem, but there is nothing listed like that in the instructions https://support.apple.com/en-au/HT202667

I found another article, https://support.apple.com/en-au/HT201356 it says to sign out of all devices that use that appleid. I can't sign out, as it is under a family sharing set up, it says 'sign out is not available due to restrictions' on her iPad. This is her only device.