How do I make pages inaccessible to the public

There are a number of pages on our website that we would like protected from the general public, and not indexed by any search engines. There is a main page that is password protected, and we would like the subpages to be accessible once the password from the top-level page has been entered.

Making the pages private is not a viable option for us, because we do not want to give the selected readers in question access to the backend, nor would they know how to navigate it.

How can we protect each page without making it password protected? What are the options for us to explore?

Thanks in advance for any suggestions.
Courtney