How can I allow/facilitate users to upload images and create a slideshow?
Sure you "could" use the 'Book' module with the child pages as you described. However that is not really going to help your users a lot. Except maybe from easily navigating from the images posted by 1 user to another user (if that is what you also want to consider)? And it does not really help I think for reducing the number of nodes to be created.

What I think is more important to consider are these items:


Add an image field to the content type you are considering using, in which you allow "multiple images". That way a single user may add all their images to "their" node. And with "permissions" it's straight forward to ensure that they can only edit their "own" node (e.g. to add more images later on). And at the same time avoid they (accidently?) add "their" images to a node of another user.
Use the Field Slideshow module to actually create a slideshow from these images.
To further simplify (for your users) the upload process of images, you may also want to consider using the IMCE module, which is an image/file uploader and browser that supports personal directories and quota.


With only these 3 items you should get pretty close to what you're looking for I think, no?