It is currently not easy to run a search under a parent folder with many sub folders. You have to manually select all the sub-folders and save that search.

When you want to vary the search term, you have to edit the search rather than just select the save search (to make it your current search) and you can go modify from there. 

Also because it is a saved search, if you create new folders under the parent, you have to remember to edit the search to include them.