Child pages. Subpages

first-level.php

//Way 1.
<?php if (have_posts()) : ?>
    <?php while (have_posts()) : the_post();
        global $post;
        $parent_page = wp_get_post_parent_id(get_the_ID()); ?>

    <?php $arg = array(
        'post_type'	      => 'page',
        'order'		      => 'ASC',
        'orderby'		      => 'title',
        'posts_per_page'  => -1,
        'post_parent'     => $parent_page,
        'post__not_in'    => array(get_the_ID())
    ); ?>


    <section class="child-pages">
        <div class="grid-container grid-container--lg">
            <div class="grid-x grid-padding-x">
                <div class="cell">
                    <h4 class="child-pages__header"><?php _e('Vi kan også helpe deg med', 'foundation'); ?></h4>
                </div>
                <?php $the_query = new WP_Query( $arg );
                if ( $the_query->have_posts() ) : ?>
                    <?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
                    <?php if (is_page() && $post->post_parent == $parent_page): ?>
                            <div class="cell medium-6 large-4">
                                <div class="child-pages__item">
                                    <a href="<?php the_permalink(); ?>" class="child-pages__link">
                                        <h5 class="child-pages__title"><?php the_title(); ?></h5>
                                    </a>
                                </div>
                            </div>
                    <?php endif ?>
                    <?php endwhile; ?>
                <?php endif; wp_reset_query(); ?>
            </div>
        </div>
    </section>

    <?php endwhile; ?>
<?php endif; ?>
//https://stackoverflow.com/questions/45869998/displaying-first-level-parent-and-co-children-of-that-parent-page