I just started using Kami today and was grappling with this issue. I thought when I got a paid account, this would fix the issue but it didn't make a difference. However, I think I discovered a simple enough fix for this issue that I hopefully can walk my little third graders through... First day of Kami had its moments but I think this will make it go smoothly. The Pro Version will help a lot I think.
First, they need to have Kami set as their default PDF viewer from Google Drive. This can be done from the Google Drive page, in settings. (Try it yourself before you try to walk someone through it.)
Second, they can favorite the folder for that class that all the math assignments go to.
Third, in case students have their files show up in tiles, have them change it to the detailed list view.
Fourth, whenever I have an assignment for them to work on, they will just go to that 'Math Folder' for my class, and then click the newest one. They can confirm the title with the List View versus the Tile View won't show it.