get pemalink of static home page

I write a plugin where I need to build full paths using path relative to the home page of my WP site.

This home page has a permalink, such as 'home_page', so these 2 urls lead to the same page:
http://www.mydomain.com
http://www.mydomain.com/home_page

So, when I want to access a child page of my home page named "child1", I need to build a path such as:
/home_page/child1

How can I get the "/home_page" value?
If