I know this is old, but this is how I did it:

$parents = get_terms(array(
  'taxonomy' => 'case_category'
  ,'hide_empty' => false
  ,'parent' => 0
  ));

  function choices($parent_cats){
    $cats = [];
    foreach ($parent_cats as $parent) {
      $cats[$parent->name] = piklist(
      get_terms('case_category', array(
        'hide_empty' => false
        ,'child_of' =>$parent->term_id
      )), array(
        'term_id','name'
      ));
    }
    return $cats;
  }
  // var_dump(choices($parents));

    piklist('field', array(
      'type' => 'select',
      'scope' => 'taxonomy',
      'field' => 'case_category',
      'label' => 'Category',
      'attributes' => array(
        'class' => 'form-control'
      ),
      'choices' => choices($parents),
    'required' => true,
    ));

I’m also having this problem in 0.9.9.9. Anyone figure out a patch?