Show child by slug, while knowing parent ID

this is how i solved my problem:

<?php   

        $pageid = $post->ID;
        $pageslug = $post->post_name;
        $pos = $post->post_parent;
        $posname = get_post_field( 'post_name', $pos );;
        
        $arg1 = array(
                'post_type'      => 'page',
                'posts_per_page' => 1,
                'pagename' => $posname."https://wordpress.stackexchange.com/".$pageslug.'/infraestructura',
                
             );
        $child = new WP_Query( $arg1 ); ?>
        <div class="wrapp">
                <?php if ($child->have_posts()) :?>
                <?php while ($child->have_posts()) : $child->the_post(); ?>
                    <?php the_title(); ?>
                    <?php the_content(); ?>
                <?php endwhile; ?>
            <?php else: ?>
                no page
            <?php endif; wp_reset_postdata();  ?>
        </div>