Template to list either sub-categories or posts

I’m looking for a template that gives either a listing of sub-categories or a listing of posts within the category if they exist. My site is set up so that there are some parent categories that contain only other categories. No category that is a parent contains posts, in other words a category either contains other categories or just posts. I’d like the template not to list posts unless they are a direct descendant of the category just above.

I’ve searched extensively trying to figure out how to do this, but haven’t found the needed help for my limited php coding skills.

Thanks for any help.