Display child pages with featured images in cards

card.scss

.card {
  background: transparent;
  box-shadow: var(--shadow-1);
  border: 1px solid var(--grey-100);
  color: var(--grey-800);
  transition: var(--ease);
  &:hover .card-figure-overlay {
    opacity: 1;
  }
  &:hover .card-title {
    background: var(--primary);
  }
  &:hover .card-readmore {
    background: var(--primary);
  }

  &-link {
    text-decoration: none;
    display: block;
    color: var(--white) !important;
  }
  &-figure {
    position: relative;
    margin: 0;
    overflow: hidden;
    &-overlay {
      background: var(--green-overlay);
      height: 404px;
      width: 100%;
      position: absolute;
      top: 0;
      left: 0;
      opacity: 0;
      z-index: 2;
    }
  }
  &-img {
    margin-top: 0 !important;
    margin-bottom: -6px !important;
    position: relative;
  }
  &-title {
    background: var(--grey-900);
    color: var(--white);
    font-weight: normal;
    font-size: 1.4rem;
    font-weight: 600;
    text-align: center;
    padding: 10px;
  }
  &-text {
    margin: 20px;
  }
  &-readmore {
    background: var(--grey-900);
    border-radius: 5px;
    color: var(--white) !important;
    display: block;
    margin: 20px;
    padding: 5px 15px;
    text-align: center;
    text-decoration: none;
    width: 120px;
  }
}

.page.iskolaink {
  .card-link {
    margin: 0;
  }
}

child-pages.php

<?php 
    global $id;
    $args = array('child_of' => $id);
    $child_pages = get_pages($args); 
?>

<div class="grid-3">
    <?php if (!empty($child_pages)): ?>
        <?php foreach ($child_pages as $key => $page_item): ?>
        <div class="card">
            <a class="card-link" href="<?php echo esc_url(get_permalink($page_item->ID)); ?>">
                <figure class="card-figure">
                    <span class="card-figure-overlay"></span>
                    <?php echo get_the_post_thumbnail($page_item->ID,'gallery'); ?>
                </figure>
                <div class="card-title"><?php echo $page_item->post_title ; ?></div>
            </a>
        </div>
        <?php endforeach ?>
    <?php endif ?>
</div>