In modifying a file in a subfolder in a child theme, do I need all the files in that sub folder in my child?

No, you should only need the files that you are going to modify.

Technically you can move all the files from the parent-theme into the child-them. When you move only the files which need changing into the child-theme then it’s a little simpler to keep track of the changes.

Also good to know when working with child-themes:
get_template_directory_uri() will always grab the path to the parent theme. & get_stylesheet_directory_uri() will always point to the path of the active theme (whether it’s parent or child)