Hi Lisa,
QA Assist has explained pretty much all the scenarios in your case pretty well.
If consent has been given, you need to use the organizer account approval to change the date of Birth if it is set under 16 years of age.
Here is something more about the organizer:
In Microsoft Family, an Organizer is the person who manages the family group and its settings.
- Organizer: Can add or remove family members, manage family safety settings, set screen-time limits, manage content/website restrictions, and view activity reports for child accounts.
I look forward to hearing from you to get the results of the troubleshooting.