Create slideshow for Folder that has many sub folders.

Definitely follow dj_paige’s advice if you want to preserve the existing subfolder hierarchy on import to Lightroom Classic. Then, when making the slide show, you do not have to use any specific folder as the source.

Instead, if you create a Collection (which is like a playlist), and use that collection as a slide show source instead of folders, that collection can use images from any combination of folders you like, or from all folders, without having to move the originals out of their subfolders.

If I am understanding you properly, you import the photos in the parent folder with the ADD option. This will import all subfolders as well, keeping the folder structure intact. Then you can create slideshows from each folder.