Hi! To change the privacy settings on a child account, you need to use the family organizer account, and the child account must be added as a member, not as an organizer.
This error can happen if the child account was added to the Microsoft Family group as an organizer. Also, if a child account leaves the Microsoft Family group, an adult account will still be required to change these settings.
To remove yourself from the family if your account was added as an organizer, sign in at account.microsoft.com/family, click the three dots on your profile, and select the option to remove consent. After that, the adult account in the family should be able to remove your account and add you back as a member, so the privacy settings can be changed.