Display children category images

Haven’t tested it, but this should do it.

    <?php

$term = get_term_by( 'slug', get_query_var( 'term' ), get_query_var( 'taxonomy' ) );
$queried_object = get_queried_object();
$term_id = get_queried_object() -> term_id;
$taxonomyName = "product-categories";
$termchildren = get_term_children( $term_id, $taxonomyName );



if (! empty ( $term_children ) ){
    foreach ($termchildren as $child) {
       $image_id = get_term_meta( $child, 'showcase-taxonomy-image-id', true );?>
       <div class="row col-xl-3 col-lg-3 col-md-3 col-sm-12 col-12 d-inline-block">
            <img class="img-fluid imgborder" src="https://wordpress.stackexchange.com/questions/287281/<?php echo wp_get_attachment_image_url( $image_id,"' ); ?>" alt="<?php echo $child->name; ?>">
            <h2><a href="<?php echo get_term_link($child, $taxonomyName); ?>"><?php echo $child->name; ?></a></h2>
        </div>
  <?php }
?>