After much time, I now understand more about the changing of a birthdate on a child Apple ID account.
In my region (US and UK) there are bands which define child, teen, and adult. They are....
- 1-12 Child
- 13-17 Teen
- 18+ Adult
Basically, you can't change a birthday from one band to another. Once a child, always a child, until the system thinks the birthday rolls into the next band.
So, once a child account is associated with a birthday under 13, it can not be changed outside the band of 0-12. But can be changed to 12 years, 364 days backwards from today.
So here is the fix: Change the birthday to today - 13 years + 1 day.
Today is August 10th, 2019, so you would use August 11, 2006. Then, the next day the account will "naturally" roll over to 13, and at that point you can change the birthday to anything in the teen band of 13-18.
I did have to also use the parental approval to approve the birthday change. So my steps were ...
- log into child account, change birthday.
- Log out of https://appleid.apple.com
- The change sends an email w/confirmation link to parent(me)
- click email link to confirm birthday change
- log into parent account and confirm change.