Is there a way to notify iParent users that a Report Card has been published?

Yes. The Publishing Dashboard provides a way to send email to all recipients of the published document. After the Report Cards (or any other published documents) have been published, click on View in the Recipients column. Just above the list of Recipients is a Notify Recipients button. Click on this to open the email editor to compose a message. More information can be found in iPASS Help > My Data > Help > Publishing Dashboard.