functions.php

<?php

/**
 * Recursively get taxonomy hierarchy
 *
 * @source http://www.daggerhart.com/wordpress-get-taxonomy-hierarchy-including-children/
 * @param string $taxonomy
 * @param int    $parent - parent term id
 *
 * @return array
 */
function get_taxonomy_hierarchy( $taxonomy, $parent = 0 ) {

	// only 1 taxonomy
	$taxonomy = is_array( $taxonomy ) ? array_shift( $taxonomy ) : $taxonomy;

	// get all direct decendents of the $parent
	$terms = get_terms( $taxonomy, array('parent' => $parent) );

	// prepare a new array.  these are the children of $parent
	// we'll ultimately copy all the $terms into this new array, but only after they
	// find their own children
	$children = array();

	// go through all the direct decendents of $parent, and gather their children
	foreach( $terms as $term ) {
		// recurse to get the direct decendents of "this" term
		$term->children = get_taxonomy_hierarchy( $taxonomy, $term->term_id );

		// add the term to our new array
		$children[ $term->term_id ] = $term;
	}

	// send the results back to the caller
	return $children;
}