I'd like to have any child page url like
automatically turns into
Could this be done in the htaccess rewrite rules or can I directly change the way WordPress stores the slug name so I could add # in front of it?
I have the parent pages loading all the child pages content in the template. It's utilizing a onepage layout with anchor links scrolling down the page to each section as you navigate. There are cases where permalinks of the child pages are exposed and that would break the onepage functionality. A child permalink should just load it's parent page and add use its slug as a an #anchor in the URL.