how i can access TextBox from ParentPage.aspx where the TextBox inside ChildPage.apx and the IFRAME is the interMediate between two pages?

hi all
how i can access TextBox from ParentPage.aspx where the TextBox inside ChildPage.apx and the IFRAME is the interMediate between two pages?
the iframe src=ChildPage.aspx , and the TextBox i want to access in ChildPage.aspx
i want to access that textbox from ParentPage.aspx By C#
how?