[ecfisa]

Hola _CALI.

Fijate si te sirve de este modo:

Código Delphi [-]
...
type
  TfrmMDIChild = class(TForm)
  private
    procedure WMSysCommand(var Msg: TMessage); message WM_SYSCOMMAND;
  ...
  end;
...

implementation

uses Unit1; // frmMDIForm

procedure TfrmMDIChild.WMSysCommand(var Msg: TMessage);
begin
  if Msg.WParam = SC_MAXIMIZE then
  begin
    ShowWindow(Handle, SW_RESTORE);
    Left   := 0;
    Top    := 0;
    Width  := frmMDIForm.ClientWidth - 4;
    Height := frmMDIForm.ClientHeight - 4
  end
  else
    inherited
end;
...

Saludos.