How do you choose to display specific posts/pages by parent url

Use the filters available in get_adjacent_post() to add a post parent condition to the WHERE clause:

function wpse_406178_adjacent_post_where_sibling( $where, $in_same_term, $excluded_terms, $taxonomy, $post ) {
    global $wpdb;

    if ( $post->post_parent ) {
        $where .= $wpdb->prepare( ' AND p.post_parent = %d', $post->post_parent );
    }

    return $where;
}

add_filter( 'get_next_post_where',     'wpse_406178_adjacent_post_where_sibling', 10, 5 );
add_filter( 'get_previous_post_where', 'wpse_406178_adjacent_post_where_sibling', 10, 5 );

Add it to your functions.php