It's so easy to use Google Drive documents with Seesaw, put it's essential that students pay attention to the image that Seesaw displays, if the image doesn't preview the Google Doc then they should know they need to change the share settings to 'anyone with the link can view' to give anyone with the link the permission to view the doc—controlling this is an essential 21st Century skill!
They need to make sure the link they share says 'sharing' at the end of the share link, not 'edit' as you can see in the example below.
This means that any parent attempting to view the doc will be prompted to sign in, which they can't do. 😕
Note: When sharing a link from a Google Doc, students will be prompted to 'Try Google Integration', my advice is NOT to do this, as it will convert the Google Doc to a PDF, which will ensure it is viewable, but will not display as well, and will not be a dynamic link, ie any changed made to the Google Doc will not be reflected in the PDF in the Seesaw post. Instead choose 'Continue' to leave the file as a Google Doc.
This also means that they're ignoring the folders in their Hapara folders in their Google drive (UoS, Reading, Writing etc) because if they put their work in those folders the share settings are automatically set for them.