refresh of parent page from child page

I have one child page which contains back button like



and the javascript is



After login there is one main page which contains dynamic menu and a iframe and inside iframe all pages are opening
my ParentPage is also opening inside iframe but when comming back from child page using the above code the ParentPage is opening inside a new window and not inside the main page iframe but i want to open it inside main page iframe when click on back button of child page