If you want to delete everything (including sub-folders) below the selected folders, make a button which runs this:
delete {filepath}*
If you want to only delete files below the selected folders, the Delete Filter can be used for that, but it sounds like you want to delete everything (based on what you're doing now to do it manually).
What I meant to say is that I select (for example) 3 folders and have the option to delete all contents, but keeping the folders, rather than going into each folder separately one by one, select all and delete it contents.
So, let's say I just want to make these these 3 folders empty.