title: Symfony 3 - static CollectionType children not showing up
body: <p>I am working on a form in Symfony 3.1.3 and want to add a dynamic amount of checkboxes using <code>CollectionType</code>. In the <code>buildForm</code>-method of my <code>Type</code>-class I am using this code to add the Checkboxes:</p>
  
  <pre><code>    $builder-&gt;add('levels', CollectionType::class, array(
          'entry_type' =&gt; CheckboxType::class,
          'mapped' =&gt; false,
      ));
  
      foreach($levels as $level) {
          $name = 'level_cb_'.$level['name'];
          $builder-&gt;get('levels')-&gt;add($name, CheckboxType::class, array(
              'label' =&gt; $level['name'],
              'required' =&gt; false,
              'mapped' =&gt; false,
          ));
      }
  </code></pre>
  
  <p>If I call <code>var_dump($builder-&gt;get('levels'));</code> immediately afterwards the checkboxes show up in the <code>children</code> and <code>unresolvedChildren</code> properties of the <code>CollectionType</code>. Later I call <code>{{ dump(form.levels) }}</code> in the associated template and it shows me that the <code>children</code> property is empty.</p>
  
  <p>Am I missing something obvious?</p>