better solution

Hi, I think, that you should do like this via post_password_required.

add_action( 'template_redirect', 'test_template_redirect', 10 );
function test_template_redirect() {

    $post_id = get_the_ID();
    $parent_ids = get_post_ancestors( $post_id );

    if ( ! $parent_ids || empty( $parent_ids ) ) {
        return;
    }

    ksort( $parent_ids );

    foreach ( $parent_ids as $parent_id ) {
        // better would be detect parent of post and redirect, but for my personal purpouse I need to redirect to top parent post.
        if ( post_password_required( $parent_id ) ) {
            wp_redirect( get_the_permalink( $parent_ids[0] ) );
            exit();
        }
    }
};