[ecfisa]

Hola.

Si entendí bien, un modo sería:

MDIChild:

Código Delphi [-]
unit Unit2;

interface

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

const
   WM_AFTER_SHOW = WM_USER + 301;

type
  TMDIChild = class(TForm)
    Edit1: TEdit;
    procedure FormShow(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
    FParametro: string;
    procedure WmAfterShow(var Msg: TMessage); message WM_AFTER_SHOW;
  public
    { Public declarations }
    property Parametro: string read FParametro write FParametro;
  end;

var
  MDIChild: TMDIChild;

implementation {$R *.dfm}

procedure TMDIChild.FormShow(Sender: TObject);
begin
  PostMessage(Self.Handle, WM_AFTER_SHOW, 0, 0);
end;

procedure TMDIChild.WmAfterShow(var Msg: TMessage);
begin
  Edit1.Text := Parametro;
end;

procedure TMDIChild.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caFree;
end;

end.

MDIForm:

Código Delphi [-]
unit Unit1;

interface

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

type
  TMDIForm = class(TForm)
    Panel1: TPanel;
    Button4: TButton;
    Button3: TButton;
    Button2: TButton;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
  private
    { Private declarations }
    procedure CallMDIChild(const nro: Integer);
  public
    { Public declarations }
  end;

var
  MDIForm: TMDIForm;

implementation {$R *.dfm}

uses Unit2;

procedure TMDIForm.CallMDIChild(const nro: Integer);
begin
  MDIChild := TMDIChild.Create(Self);
  MDIChild.Parametro := IntToStr(nro)
end;

procedure TMDIForm.Button1Click(Sender: TObject);
begin
  CallMDIChild(111);
end;

procedure TMDIForm.Button2Click(Sender: TObject);
begin
  CallMDIChild(222);
end;

procedure TMDIForm.Button3Click(Sender: TObject);
begin
  CallMDIChild(333);
end;

procedure TMDIForm.Button4Click(Sender: TObject);
begin
 CallMDIChild(444);
end;

end.

Salida del ejemplo:


Saludos