WordPress, Nested queries in get_pages

I have a page structure like this.

    Portfolio
    - Residential
    - - Resident 1
    - - Resident 2
    - - Resident 3
    - Commercial
    - - Commercial 1
    - - Commercial 2
    - - Commercial 3

The grandchild pages Resident 1 – Commercial 3 have post thumbnails

I have a template for the Portfolio page where i would like to display the ‘Residential’ and ‘Commerical’ pages with their child pages like so.

<div class="grid">
    <h2>Residential</h2>
    <ul>
        <li> Resident 1 post_thumbnail</li>
        <li> Resident 2 post_thumbnail</li>
        <li> Resident 3 post_thumbnail</li>
    </ul>
</div>
<div class="grid">
    <h2>Commercial</h2>
    <ul>
        <li> Commercial 1 post_thumbnail</li>
        <li> Commercial 2 post_thumbnail</li>
        <li> Commercial 3 post_thumbnail</li>
    </ul>
</div>

I’m using get_pages and I have the div’s created for ‘Residential’ and ‘Commercial’

<?php

$portfolio_sections = array(
    'post_type'   => 'page',
    'child_of'    => $post->ID,
    'sort_column' => 'menu_order',
    'sort_order'  => 'ASC',
    'parent'      => $post->ID
);

$sections = get_pages($portfolio_sections);

foreach ($sections as $section) { ?>
    <div class="grid">
        <h2><?php echo $section->post_title; ?></h2>

        //Create Child pages

    </div><!--.grid-->
<?php } ?>

My problem is creating the child pages in the ‘ul’ list

I tried using a second foreach loop but this didn’t work and I don’t know if this is the correct way

<?php

$portfolio_sections = array(
    'post_type'   => 'page',
    'child_of'    => $post->ID,
    'sort_column' => 'menu_order',
    'sort_order'  => 'ASC',
    'parent'      => $post->ID
);

$sections = get_pages($portfolio_sections);

foreach ($sections as $section) { ?>
    <div class="grid">
        <h2><?php echo $section->post_title; ?></h2>
        <ul class="imageGrid">
            <?php
            $portfolio_pages = array(
                'post_type'   => 'page',
                'child_of'    => $section->ID,
                'sort_column' => 'menu_order',
                'sort_order'  => 'ASC',
                'parent'      => $section->ID
            );

            $pages = get_pages($portfolio_pages);

            foreach ($pages as $page) { ?>
                <li>
                    <a href="<?php echo get_the_permalink($page->ID); ?>"><?php echo get_the_post_thumbnail($page->ID, "thumbnail"); ?>
                        <span><?php echo get_the_title($page->ID); ?></span></a>
                </li>
            <?php } ?>
        </ul>
    </div><!--.grid-->
<?php } ?>