C#

private delegate bool EnumChildProc(IntPtr hwnd, IntPtr lParam);

[DllImport("User32.dll")]
private static extern bool EnumChildWindows(IntPtr hWndParent, EnumChildProc lpEnumFunc, IntPtr lParam);

private IEnumerable<Form> GetChildrenSortedByZOrder()
{
    List<IntPtr> handles = new List<IntPtr>();
    if (IsHandleCreated)
    {
        EnumChildWindows(Handle,
            (hWnd, lparam) =>
            {
                handles.Add(hWnd);
                return true;
            }, IntPtr.Zero);
    }
    List<Form> children = new List<Form>(handles.Count);
    foreach (IntPtr handle in handles)
    {
        Form form = FromHandle(handle) as Form;
        if (form != null)
            children.Add(form);
    }
    return children;
}

private void ShowMdiChildren() {
  // 'log' is a TextWriter
  log.WriteLine("MdiClient control collection");
  log.WriteLine("============================");
  foreach (Control c in this.Controls) {
    if (c is System.Windows.Forms.MdiClient) {
      System.Windows.Forms.Control.ControlCollection mdiColl = c.Controls;

      // Write:  Index  Text  Name  Type
      foreach (Control mdiChild in mdiColl) {
        log.WriteLine("{0,-2} {1, -20} {2, -16} {3}", mdiColl.GetChildIndex(mdiChild), mdiChild.Text, mdiChild.Name, mdiChild.GetType());
      }

      // equivalent code
      //for (Int32 index = 0; index < mdiColl.Count; index++) {
      //  Control child = mdiColl[index];
      //  log.WriteLine("{0,-2} {1, -20} {2, -16} {3}", index, child.Text, child.Name, child.GetType());
      //}
    }
  }
  log.WriteLine();
}

private void RotateZOrder() {
  foreach (Control c in this.Controls) {
    if (c is MdiClient) {
      // send index 0 (front) to back
      Control.ControlCollection mdiColl = c.Controls;
      if (mdiColl.Count >= 2) {
        mdiColl.SetChildIndex(mdiColl[0], mdiColl.Count - 1);
      }
    }
  }
}