//child form
type
  TMyRec = packed record
    s1: String;
    s2: String;
  end;

  TForm2 = class(TForm)
  <skiped>
  private
    FMyRec: TMyRec;
    procedure SetMyRec(AValue: TMyRec);
  public
    property MyRec: TMyRec read FMyRec write SetMyRec;
  end;

procedure TForm2.SetMyRec(AValue: TMyRec);
begin
  if (FMyRec.s1 <> AValue.s1) then FMyRec.s1:= AValue.s1;
  if (FMyRec.s2 <> AValue.s2) then FMyRec.s2:= AValue.s2;
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
  MyRec:= Default(TMyRec);
end;

//parent form
procedure TForm1.Button1Click(Sender: TObject);
var
  tf: TForm2;
begin
  tf:= TForm2.Create(Self);
  try
    tf.MyRec.s1:= 'aaaaaa';
    tf.MyRec.s2:= 'ssssss';
    tf.ShowModal;
  finally
    FreeAndNil(tf);
  end;
end;