Are these two GridView in the same page? If so, you can set an updatepanel to wrap the parent gridview and set the button which is sticking on the child gridview as the trigger. Every button clicking will update the parent gridview.
If they are not in the same page, and the page which contains child gridview is the child page of that, you can use javascript to refresh the parent page.
Page.RegisterStartupScript("refresh", "<script language=javascript>window.opener.location.reload;</script>");