Gravity Form Dynamic ajax category + child dropdown

//Dynamic category dropdown
add_filter("gform_pre_render_2", "monitor_single_dropdown");
function monitor_single_dropdown($form){

    global $wpdb;
    $nomos_table = $wpdb->prefix . 'nomous';
    $wp_nomos_search = $wpdb->get_results("SELECT nomos_ID, nomos_Title FROM $nomos_table ORDER BY nomos_ID ASC");

    //Creating drop down item array.
    $items = array();

    //Adding nomos to the items array
    foreach ( $wp_nomos_search as $nomos ){
    $is_selected = $nomos->nomos_Title == "Θεσσαλονίκης" ? true : false;
    $items[] = array("value" => $nomos->nomos_ID, "text" => $nomos->nomos_Title, "isSelected"=> $is_selected);
    }

    //Adding other... item
    $items[] = array("text" => "Other...", "value" => "0");

     //Adding items to field id 28
        foreach($form["fields"] as &$field)

            if($field["id"] == 28){
                $field["type"] = "select";
                $field["choices"] = $items;
}

?>
<script type="text/javascript">

jQuery(document).ready(function(){

    jQuery('#input_2_28').change(function(){
            var mainCat=jQuery('#input_2_28').val();

            // call ajax
             jQuery("#input_2_31").empty();
                jQuery.ajax({
                    url:"<?php bloginfo( 'wpurl' ); ?>/wp-admin/admin-ajax.php",
                    type:'POST',
                    data:'action=monitor_children_dropdown&main_catid=' + mainCat,

                     success:function(results)
                         {
            //  alert(results);
               jQuery("#input_2_31").append(results);
                                }
                           });
                  }
            );
});

</script>

<?php
return $form;
}

//ajax callback function
function monitor_children_dropdown() {
if(isset($_POST['main_catid'])) {

$parentCat=$_POST['main_catid'];
global $wpdb;
$dimoi_table = $wpdb->prefix . 'dimoi';
$wp_dimos_search = $wpdb->get_results("SELECT dimos_ID, dimos_Title FROM $dimoi_table WHERE nomos_ID=$parentCat ORDER BY dimos_ID ASC");

              //Creating drop down item.
              foreach ($wp_dimos_search as $dimos) {
                $option .= '<option value="'.$dimos->dimos_ID.'">';
                $option .= $dimos->dimos_Title;
                $option .= '</option>';
              }
               //   Adding other... item
              echo '<option value="0" selected="selected">Select...</option>'.$option;
      die();
 } // end if
}

add_action( 'wp_ajax_nopriv_monitor_children_dropdown', 'monitor_children_dropdown'); //keep for people who allow post before registration
add_action( 'wp_ajax_monitor_children_dropdown', 'monitor_children_dropdown');