Get WP ACF-Plugin Field value from child category

functions.php

<?php  

  /*
  *  Get Advanced Custom Field value from child category 
  */

  $cats = get_categories('child_of='.get_query_var('cat')); 

  foreach ($cats as $cat) :

  $args = array(
    'posts_per_page' => 3, // max number of post per category
    'category__in' => array($cat->term_id)
  );

  $my_query = new WP_Query($args); 

    if ($my_query->have_posts()) : 

    echo '<h3>'.$cat->name.'</h3>';
    
    // 'background_image' Custom Field 
    the_field('background_image', 'category_' . $cat->term_id); 
    
    echo $cat->description;

    while ($my_query->have_posts()) : $my_query->the_post(); ?>     
      <a href="<?php the_permalink() ?>"><?php the_title(); ?></a>    <br />  
    <?php endwhile; ?>

    <?php else : 
    echo 'No Posts for '.$cat->name;                
    endif; 

  endforeach; 

?>