Portfolio Template using Static Child Pages

content-portfolio.php

<?php
/**
 * The template used for displaying portfolio items.
 */
?>

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

	<a href="<?php the_permalink(); ?>" rel="bookmark">
		<?php the_post_thumbnail( 'medium' ); ?>
	</a>

	<header class="entry-header">
		<?php the_title( sprintf( '<h2 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h2>' ); ?>
	</header>

</article>

functions.php

/**
 *  Displays static pages in a grid layout.
 */
function theme_slug_child_page_grid() {

	// Get current page ID.
	$page_id = get_the_ID();

	// Get Child Pages from Database.
	$query_arguments = array(
		'post_type'      => 'page',
		'post_parent'    => (int) $page_id,
		'posts_per_page' => -1,
		'no_found_rows'  => true,
		'orderby'       => 'menu_order title',
		'order'         => 'ASC',
	);
	$portfolio_query = new WP_Query( $query_arguments );

	// Check if there are child pages.
	if ( $portfolio_query->have_posts() ) : ?>

		<div class="child-page-grid">

			<?php while ( $portfolio_query->have_posts() ) : $portfolio_query->the_post();

				get_template_part( 'template-parts/content', 'portfolio' );

			endwhile; ?>

		</div>

	<?php
	endif;

	// Reset Postdata.
	wp_reset_postdata();
}

style.css

/*======================================
Theme Name: TwentySixteen Child
Description: Child Theme of TwentySixteen
Version: 1.0
Author: ThemeZee
Author URI: http://themezee.com
Template: twentysixteen
License: GNU General Public License v2.0
License URI: http://www.gnu.org/licenses/gpl-2.0.html
======================================*/


/* You can start adding your own styles here. Use !important to overwrite styles if needed. */
.child-page-grid {
    display: flex;
    flex-wrap: wrap;
    margin-right: -2em;
}

.child-page-grid .type-page {
    width: 33.33333333333%;
    box-sizing: border-box;
    padding-right: 2em;
    padding-bottom: 2em;
}

.child-page-grid .type-page .entry-title {
    margin: 1em 0 0;
    font-size: 20px;
    font-size: 1.25rem;
}

template-portfolio.php

<?php
/**
 * Template Name: Portfolio
 *
 * @package Twenty Sixteen Child
 */

get_header(); ?>

<div id="primary" class="content-area">
	<main id="main" class="site-main" role="main">
		<?php
		// Start the loop.
		while ( have_posts() ) : the_post(); ?>

			<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

				<header class="entry-header">
					<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
				</header><!-- .entry-header -->

				<?php twentysixteen_post_thumbnail(); ?>

				<div class="entry-content">
					<?php
					the_content();

					// Display Portfolio.
					theme_slug_child_page_grid();
					?>
				</div><!-- .entry-content -->

			</article>

			<?php
			// If comments are open or we have at least one comment, load up the comment template.
			if ( comments_open() || get_comments_number() ) {
				comments_template();
			}

			// End of the loop.
		endwhile;
		?>

	</main><!-- .site-main -->

	<?php get_sidebar( 'content-bottom' ); ?>

</div><!-- .content-area -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>