A child portal in DotNetNuke consists of data in the same database and two folder (with associated files).  You cannot selectively backup data in the database, but anytime you take a backup it will capture the database data for all the sites in the instance.

With regard to the files, the first folder is created at the website root and contains only a default.aspx file. This folder is really only there to ensure that a browser reaches the site correctly, since the "child" is named from the parent in a way that looks like a subdirectory (www.parent.com/child).  This is why the name of the folder matches your site name.

The second folder, the one with a number, is created under the /Portals folder and represents the "portal root" for files managed within that portal.  It does not HAVE to be located there, or named that way... but this is the default and is established at creation of the site.  When you create a site using the admin screens, it asks for "home directory"... this is what it is creating.  All the links in your portal and files managed with the file manager will live within this folder.

Scott Willhite, Co-Founder DNN