The problem is that ‘post_parent’ is not a public query var, i.e. you can use it only from code.
To be able to use it from the URL, you just need to add these lines of code:
function make_post_parent_public_qv() {
if ( is_admin() )
$GLOBALS['wp']->add_query_var( 'post_parent' );
}
add_action( 'init', 'make_post_parent_public_qv' );