Is there a away to show the unassigned sub-tasks of assigned parent tasks?

Context: I have created a Project location that 3 users share. An extensive list of clients, as parent tasks, and the corresponding assets for that client as sub-tasks. Within those sub-tasks (the assets) is a list of milestones for the asset's completion.

The users would like to use the project location to see their own clients. Filtering by assignee would be logical, but they are only assigned to the parent task(the client) and the sub-tasks (the assets) not the sub-sub task (the milestones). When filtering by the assignee their sub-sub task disappears. When also selecting unassigned in the filtering, the other user's tasks populate. 

Question:

Is there a way to filter negatively- not include x and y.

Or alternatively is there a way to make show the unassigned sub-tasks of assigned parent tasks?

The only workaround I can think of is filtering by author, but that would require them to recreate what I originally created.
Thank you for any suggestions.