How can I create a report in Quickbase that aggregates multiple rows into a single line per record?

np,

The first step is that on the child Physician Credentials record you need to create a single field called perhaps [Physician Credential for Roll Up] which nicely labels and appends together all the fields for Roll up.  (Post back if you need help with that)

Then you will make a Combined Text Summary field (ie on the relationship) of [Physician Credential for Roll Up] called [Combined Text Physician Credentials for Roll Up]

Then because that will look terrible on a report, make a new formula text field on the parent field called [Physician Credentials Summary] with a formula of

SearchAndReplace(ToText([Combined Text Physician Credentials for Roll Up]), " ; ", "\n")

That will replace the field which has a bubble appearance with a one that nice vertical list of the Physician Credentials in one field suitable for a column on a report.