[erikrocha]

Hola Neftali, movorack y escafandra. Gracias por sus respuestas. Tienen razón, entonces aquí les muestro todo el código y también les adjunto los archivos, si lo ejecutan verán el error. Una vez más gracias por su tiempo.

main (MDIForm)

Código Delphi [-]
unit main;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  Tfrm_main = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  frm_main: Tfrm_main;

implementation

{$R *.dfm}

uses
  pos;

procedure Tfrm_main.Button1Click(Sender: TObject);
var
  pos: Tfrm_pos;
begin
  pos := Tfrm_pos.Create(self);
end;

end.

pos (MDIchild)

Código Delphi [-]
unit pos;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  Tfrm_pos = class(TForm)
    Label1: TLabel;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  frm_pos: Tfrm_pos;

implementation

{$R *.dfm}

uses
  search;

procedure Tfrm_pos.Button1Click(Sender: TObject);
var
  search: Tfrm_search;
begin
  search := Tfrm_search.Create(self);
end;

end.

search (MDIchild)

Código Delphi [-]
unit search;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  Tfrm_search = class(TForm)
    Edit1: TEdit;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  frm_search: Tfrm_search;

implementation

{$R *.dfm}

uses
  pos;

procedure Tfrm_search.Button1Click(Sender: TObject);
begin
  frm_pos.Label1.Caption := edit1.Text;
end;

end.