procedure ShowChild(MainForm : TForm; InstanceClass: TFormClass; var Reference);
var
  Instance: TForm;
begin
  Screen.Cursor:= crHourglass;
  LockWindowUpdate(MainForm.Handle);
  if not IsChildFormExist(InstanceClass) then
    try
      Instance:= TForm(InstanceClass.NewInstance);
      TForm(Reference):= Instance;
      try
        Instance.Create(MainForm);
        if (Instance as TForm).FormStyle = fsNormal then
        begin
          (Instance as TForm).FormStyle := fsMdiChild;
          (Instance as TForm).Visible := True;
        end;
      except
        TForm(Reference):= nil;
        Instance.Free;
        raise;
      end;
    finally
      Screen.Cursor:= crDefault;
    end
  else
    with TForm(Reference) do begin
      if WindowState = wsMinimized then WindowState:= wsNormal;
      BringToFront;
      Screen.Cursor:= crDefault;
      SetFocus;
    end;
 
  LockWindowUpdate(0);
end;

procedure TfrmPrincipal.btnServicoClick(Sender: TObject);
begin
  ShowChild(Self, TFrmCLiente, FrmCLiente);
end;