Show admin help message across custom post type parent and child posts

You’ll need to check the $pagenow variable and the post type of the post being edited. It will look something like this:

function wpse_75224_admin_notices() {
    global $pagenow;

    $is_edit_custom_post_type = ( 'post.php ' == $pagenow && 'my_custom_post_type' == get_post_type( $_GET['post'] ) );
    $is_new_custom_post_type = ( 'post-new.php' == $pagenow && 'my_custom_post_type' == $_GET['post_type'] );
    $is_all_post_type = ( 'edit.php' == $pagenow && 'my_custom_post_type' == $_GET['post_type'] );

    if ( $is_all_post_type || $is_edit_custom_post_type || $is_new_custom_post_type ) {
        echo "Your message.";
    }
}

add_action( 'admin_notices', 'wpse_75224_admin_notices' );