The child no longer has an iPhone, How can I remove an under 15 yo child's old account from my family plan. I was going to transfer to another family and transfer her as well but she doesn't remember the password and I am unable to transfer my account to another family without removing her first.
If she forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if she can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.
You can try deleting the account if you can recover the password.
How to delete your Apple ID account. *
What to do after you change your Apple ID or password - Apple Support -
transfer a child to another family account
