Using a computer or laptop, head to your My T-Mobile account and log in using the parent phone number and the password you created.
The contact will say “Error adding this line”—go ahead and click the ‘Delete’ button directly under the contact’s thumbnail photo.
Once the contact has been deleted, click on the ‘Settings’ link in the top left corner of the screen. From there, click on the link that says “Add a family member” (under the section titled ‘Family members’).
Select the child line that you just deleted, enter the name of the contact, and click 'Add'.
That’s it! Your child line has been successfully added to FamilyWhere.