<zk>
  <button label="invalidate treechildren" onClick="tc.invalidate();"/>
  <tree>
    <treechildren id="tc">
      <treeitem label="item1"/>
      <treeitem label="item2"/>
      <treeitem label="item3"/>
      <treeitem label="item4"/>
      <treeitem label="item5"/>
    </treechildren>
  </tree>
</zk>