Ajax con combobox dependientes Drupal 7

http://atur-lalai.blogspot.com/2011/10/drupal-7-ajax-dependent-drop-down.html

function filtro_buenaspracticas_form($form, &$form_state, $month, $year, $area)
{
    $start_date = format_date(mktime(0,0,0,$month,1,$year),'custom', 'Y-m-d');
        //get the list of locations from taxonomy to use in the dropdown
        $dropdown_source = taxonomy_get_tree(141, $parent = 0, $max_depth = 1, $load_entities = FALSE);
        //print_r($dropdown_source);
        $dropdown_array = array('0' => '--none--');
        foreach ($dropdown_source as $item) {
            $key = $item->tid;
            if ($item->depth == 0){
                $value = $item->name;
            }else{
                $value = '--'.$item->name;
            }
            $dropdown_array[$key] = $value;
        }
        $info = node_load(775);
        //print_r($info);
    $form = array();
        $form['conte'] = array(
            '#prefix' => '<div class="contenido-pedeterminado">'. $info->body['und'][0]['value'].'',
            '#suffix' => '</div>',
        );
   
        $form['area'] = array(
            '#key_type' => 'associative',
            '#type' => 'select',
            '#options' => $dropdown_array,
            '#title' => 'Empresa',
            '#default_value' => $area,
            '#ajax' => array(
                'callback' => 'ajax_dropdown_callback',
                'wrapper' => 'second-option',),
            );
        $selected = isset($form_state['values']['area']) ? $form_state['values']['area'] : key($dropdown_array);
        $form['select2'] = array(
            '#options' => _select2_dropdown_options($selected),
            '#default_value' => isset($form_state['values']['select2']) ? $form_state['values']['select2'] : '',    
            '#prefix' => '<div id="second-option">',
            '#suffix' => '</div>',
            '#title' => 'Area',
            '#type' => 'select',
          );
        $form['month'] = array(
        '#type' => 'date_select',
        '#date_format'    =>    'F',
        '#date_type' => DATE_DATETIME,
        //'#date_year_range' => '-0:+1',
        '#default_value' => $start_date,
    );
    $form['year'] = array(
        '#type' => 'date_select',
        '#date_format'    =>    'Y',
        '#date_type' => DATE_DATETIME,
        '#date_year_range' => '-5:-0',
        '#default_value' => $start_date,
    );
       
    $form['submit'] = array(
        '#type' => 'submit',
        '#value' => t('Filter'),
        '#submit' => array('datos_buenaspracticas_submit'),
    );
    $form['reset'] = array(
        '#type' => 'submit',
        '#value' => t('Reset'),
        '#submit' => array('reset_datos_buenaspracticas_submit'),
       
    );
    return $form;
}

//Funciones
function _select2_dropdown_options($key=Null) {
    $tid = $key;
     $a = taxonomy_get_children($tid);
    //print_r($a);
    $dropdown_array_2 = array('0' => '--none--');
    //$dropdown_array_2 = array("2"=>'demo 2');
    foreach ($a as $item) {
            $key = $item->tid;
            $value = $item->name;
           /*  if ($item->depth == 0){
                $value = $item->name;
            }else{
                $value = '--'.$item->name;
            }*/
           $dropdown_array_2[$key] = $value;
    }
      
    return $dropdown_array_2;
}

function ajax_dropdown_callback($form, &$form_state) {
     return $form['select2'];
}