From the Name your devices section, click the 'Role' dropdown menu next to the associated mobile number.
Select the desired option:
Only 2 devices can be set as parent and the Account Owner is the primary parent.
If the device is currently set to a Child role, first change the role to 'Not assigned' and save then select the option below.
To enable a device as a parent, the mobile number must be an Account Manager in My Verizon.