The details relating to the sub-pages that you can insert into parent pages comes directly from the Info tab of each of the sub-pages.
The following image shows the places on the Info tab where the title and description come from for each of the sub-pages. In the bottom left of this image is an extract from a page containing a bulleted list with descriptions for each of the sub-pages.
As you can see the Page Title becomes the title and the link to the sub-page and the Description for site map and meta tags becomes the short description. This saves you having to enter another description each time you add a new sub-page as the description you enter on the Info tab should be just fine.