Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

// Write this code in your MDI Child Window unit. 
// Schreibe diesen Code in die Unit des MDI Child Fensters: 

type
   TfrmMyMDIForm = class(TForm)
   public
     procedure WMSize(var Msg: TWMSIZE); message WM_SIZE
   end;

 implementation

 procedure TfrmMyMDIForm.WMSize(var Msg: TWMSize);
 begin
   if Msg.SizeType = SIZE_MINIMIZED then
     ShowWindow(Handle, SW_HIDE);
 end;