If you have a personal Gmail account, you could export the gradebook to an excel file and upload it into Google Drive.
From there, set the sharing permissions to anyone with the link. On your Moodle course, you can create a URL resource which links to this document.
Enroll the parents as participants or students in the course, and then create a group with all of the parents in it. Then put that group in a grouping, and restrict access to the URL to just the users in that grouping.
It's a bit of an inelegant solution, as you will need to export and re-upload the gradebook in drive anytime you make a change. You could set it up as a quarterly report type item, to make this a bit easier on you.
My two cents, for what it's worth