Обход PHP RecursiveIterator

У меня есть структура, представляющая форму, и я хочу повторить ее с помощью RecursiveIterator. Проблема в том, что это возвращает только вопросы верхнего уровня. Что я делаю не так?

Вся форма:

class Form implements RecursiveIterator{
    private $id;
    private $caption;
    private $other_text;
    private $questions = array();
    private $current;

    private function __construct(DibiRow $row){
        $this->id = $row->id;
        $this->caption = $row->caption;
        $this->other_text = $row->other_text;
        $this->loadQuestions();
    }

    private function loadQuestions(){
        $questions = dibi::query('SELECT * FROM cyp_questions WHERE form_id = %i AND parent_id IS NULL', $this->id);
        while($question = $questions->fetch()) $this->questions[] = new Question($question->question_id, $question->type, $question->caption, $question->other_text, $question->triggers_unique == 1);
    }

    /**
     * @throws InvalidArgumentException
     * @param $id
     * @return Form
     */
    public static function loadById($id){
        $form = dibi::query('SELECT * FROM cyp_forms WHERE id = %i', $id)->fetch();
        if($form === false) throw new InvalidArgumentException('Form with id '.$id.' was not found.');

        return new Form($form);
    }

    /**
     * @throws FormFieldException
     * @return bool
     */
    public function validate($postfields){

    }

    public function getQuestions(){
        return $this->questions;
    }

    public function getChildren(){
        return $this->questions[$this->current];
    }

    public function hasChildren(){
        return count($this->questions) > 0;
    }

    public function current(){
        return $this->questions[$this->current];
    }

    public function key(){
        return $this->current;
    }

    public function next(){
        $this->current++;
    }

    public function rewind(){
        $this->current = 0;
    }

    public function valid(){
        return isset($this->questions[$this->current]);
    }
}

Вопрос:

class Question implements RecursiveIterator{
    private $id;
    private $type;
    private $answers = array();
    private $subquestions = array();
    private $other_text;
    private $triggers_unique;
    private $caption;
    private $current = 0;


    public function __construct($id, $type, $caption, $other_text = null, $triggers_unique = false){
        $this->id = $id;
        $this->type = $type;
        $this->caption = $caption;
        $this->other_text = $other_text;
        $this->triggers_unique = $triggers_unique;  
        $this->setSubQuestions();   

    }

    private function setSubQuestions(){
        $questions = dibi::query('SELECT * FROM cyp_questions WHERE parent_id = %i', $this->id);
        while($question = $questions->fetch()) $this->subquestions[] = new Question($question->question_id, $question->type, $question->caption, $question->other_text, $question->triggers_unique == 1);
    }

    public function getOtherText(){
        return $this->other_text;
    }

    public function getCaption(){
        return $this->caption;
    }

    public function addAnswer($answer){
        $this->answers[] = $answer;
    }

    public function getChildren(){
        return $this->subquestions[$this->current];
    }

    public function hasChildren(){
        return count($this->subquestions) > 0;
    }

    public function current(){
        return $this->subquestions[$this->current];
    }

    public function key(){
        return $this->id;
    }

    public function next(){
        ++$this->current;
    }

    public function rewind(){
        $this->current = 0; 
    }

    public function valid(){
        return isset($this->subquestions[$this->current]);
    }

    public function getAnswers(){
        return $this->answers;
    }


}

Итерация:

$form = Form::loadById(1);

foreach($form as $question){
    echo $question->getCaption().'<br />';  
}