get parent record ID for use on child page

If I have tbl_parent and tbl_children
tbl_parent

• id
  
• name
  
• address
  tbl_children
  
• id
  
• parent_ID
  
• name
  
• address
  from the tbl_parent list page I click on the link to add/edit/delete children for this parent
  I have placed some php code on the children list page and I need to get the parent ID added to my php code so I can use it in an external page I'm building.
  I tried pulling it from the initial URL and that worked, however after you add/edit a child record the URL changes to a list page url without any GET variables so then my PHP fails.