When you click Posts > Add New, you create a new post that will show up in the main page by default. But that's not necessarily so: you can set your main page to display a static page instead of your latest posts.
Posts are also accessible by category via the sidebar, if you activate the Categories widget. So you could have a "Book Reviews" post category instead of a BR page. (You can even have that category as a page tab in the header, if you wish.)
Parent and child refers to pages, not posts: Pages > Add New. The page editor includes an Attributes module, which includes assigning a parent. If you don't see that module, click on "Screen Options" (while you are on the page editor screen) to activate it.
I'm not sure I understand your last question.