After a Parent of Candidate or a New Parent fills out an inquiry for one child, their basic information is put into the system. To add the second child and avoid duplicates in the system, when the parent logs in there will be a button on the right side that says Apply Another Child. The user must have a Parent Role and have a child that is currently in the candidate process. 

User-added image

Additionally, if a parent has no current candidate-children, the 'Apply Another Child' button won't be visible.
For a parent to access the interface to apply another child, have them paste the 'apply' URL (which can be found via Enrollment Management > Admissions > Admissions Setup > Application Forms > Login Setup), after they login.
They will then see the dialogue to apply a new candidate.