menu-thumbnails

/**
* Add featured image thumbnails to children of menu items with class menu-thumbs
*
* @param array $items List of menu objects (WP_Post).
* @param array $args  Array of menu settings.
* @return array
*/
function add_images_to_special_submenu( $items ) {
  $special_menu_parent_ids = array();

  foreach ( $items as $item ) {
    if ( in_array( 'menu-thumbs', $item->classes, true ) && isset( $item->ID ) ) {
      $special_menu_parent_ids[] = $item->ID;
    }

    if ( in_array( $item->menu_item_parent, $special_menu_parent_ids ) && has_post_thumbnail( $item->object_id ) ) {
      $item->title = sprintf(
        '%1$s %2$s',
        get_the_post_thumbnail( $item->object_id, 'thumbnail', array( 'alt' => esc_attr( $item->title ) ) ),
        $item->title
      );
    }
  }

  return $items;
}

add_filter( 'wp_nav_menu_objects', 'add_images_to_special_submenu' );