Add parent class to top level terms that have children in wp_list_categories for a custom taxonomy using a custom Walker class.

gistfile1.php

<?php
class My_Walker_Category extends Walker_Category {

/**
* @see Walker::start_el()
* @since 2.1.0
*
* @param string $output Passed by reference. Used to append additional content.
* @param object $category Category data object.
* @param int $depth Depth of category in reference to parents.
* @param array $args
*/
function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {

  global $wp_query;
extract($args);

$cat_name = esc_attr( $category->name );
$cat_name = apply_filters( 'list_cats', $cat_name, $category );
$link = '<a href="' . esc_url( get_term_link($category) ) . '" ';
		if ( $use_desc_for_title == 0 || empty($category->description) )
			$link .= 'title="' . esc_attr( sprintf(__( 'View all posts filed under %s' ), $cat_name) ) . '"';
		else
			$link .= 'title="' . esc_attr( strip_tags( apply_filters( 'category_description', $category->description, $category ) ) ) . '"';
		$link .= '>';
		$link .= $cat_name . '</a>';

		if ( !empty($feed_image) || !empty($feed) ) {
			$link .= ' ';

			if ( empty($feed_image) )
				$link .= '(';

			$link .= '<a href="' . esc_url( get_term_feed_link( $category->term_id, $category->taxonomy, $feed_type ) ) . '"';

			if ( empty($feed) ) {
				$alt = ' alt="' . sprintf(__( 'Feed for all posts filed under %s' ), $cat_name ) . '"';
			} else {
				$title = ' title="' . $feed . '"';
				$alt = ' alt="' . $feed . '"';
				$name = $feed;
				$link .= $title;
			}

			$link .= '>';

			if ( empty($feed_image) )
				$link .= $name;
			else
				$link .= "<img src='$feed_image'$alt$title" . ' />';

			$link .= '</a>';

			if ( empty($feed_image) )
				$link .= ')';
		}

		if ( !empty($show_count) )
			$link .= ' (' . intval($category->count) . ')';

		if ( 'list' == $args['style'] ) {
			$output .= "\t<li";

			$class = 'cat-item cat-item-' . $category->term_id;

			/** If the current category is a top level element and it has children, add a parent class */
			if($category->category_parent == 0 && $category->hasChildren == true )
				$class .= ' parent-item';
			if ( !empty($current_category) ) {
				$_current_category = get_term( $current_category, $category->taxonomy );
				if ( $category->term_id == $current_category )
					$class .=  ' current-cat';
				elseif ( $category->term_id == $_current_category->parent )
					$class .=  ' current-cat-parent';
			}
			$output .=  ' class="' . $class . '"';
			$output .= ">$link\n";
		} else {
			$output .= "\t$link<br />\n";
		}
	}

	function display_element ($element, &$children_elements, $max_depth, $depth = 0, $args, &$output)
	{
		// check, whether there are children for the given ID and append it to the element with a (new) ID
		$element->hasChildren =  !empty($children_elements[$element->term_id]);

		return parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
	}
}

Thanks!