使用Codeigniter的相关下拉列表

This is my model:

public function get_site_lang()
        {
            $query = $this->db->query("SELECT id_lang,name from LANGUAGE");
            return $query->result_array();
        }

        public function get_compo_list($language)
        {
            $query = $this->db->query("SELECT m.id_menu,name from lang_menu l JOIN MENU m ON l.menu_id=m.id_menu
                                       WHERE m.parent_id is NULL and l.lang_id=$language");
            return $query->result_array();
        }

        public function get_compo_cat_list($id,$language)
        {
            $query = $this->db->query("SELECT m.id_menu,name from lang_menu l JOIN MENU m ON l.menu_id=m.id_menu
                                           WHERE m.parent_id=$id and l.lang_id=$language");
            return $query->result_array();
        }

This is my view:

<div class="form-group">
    <label>Select Language</label>
    <select class="form-control" name="language" required>
        <?php
            foreach ($lang as $l) {
            $selected = ($l['id_lang'] == $this->input->post('lang_id')) ? ' selected="selected"' : null;
                                                echo '<option value="'.$l['id_lang'].'" '.$selected.'>'.$l['name'].'</option>';

                                            }
                                            ?>
                                        </select>
                                    </div>






                                  <div class="form-group">
                                  <label>Select Parent</label>
                                  <select name="id_menu" class="form-control" id="id_menu">

                                      <?php
                                      foreach ($compo as $co)
                                      {
                                          echo "<option value=".$co['id_menu'].">".$co['name']."</option>";

                                      }
                                      ?>

                                    </select>
                                </div>

                                    <div class="form-group">
                                        <label>Select Child</label>
                                    <select class="form-control" name="type">
                                        <option value="">Select A Child</option>

                                    </select>
                                        </div>

How can I show in the "Select A Child" options that I should have after public function get_compo_cat_list($id,$language) run? I'm missing the point after trying with (Ajax) and (JavaScript) and it is not working. What I am missing here?