Sharepoint 2010: Sharing document libraries from parent site to child site

Here is my site and library structure from the parent site

Sites
 child site A
 child site B
 child site C

Libraries
 test documents


I would like to create document libraries on the parent that I can then share with specific children.

I tried to add the groups that were created with "child site A" to the permissions of "test documents" but i was still not able to add "test documents" as a web part for "child site A"

How can this be achieved?

Thank you