How to retrieve a value off a related opportunity in a report

We are using the Parent/Child (Sub-Opportunity) relationship for opportunities.  On a Parent opportunity we have a field I'll call Proposal Number.  On a report we only display the Child opportunity records.  But on the report I'd like to include the Proposal Number that is on the Parent opportunity.  Is there a way in a report to pull the Proposal Number off the Parent opportunity.  The Child opportunity record has the Parent Opportunity Id so I thought there might be a way to retrieve the information.