Сворачивание MDIChild окна

  private
    procedure WMGetSysCommand(var Msg: TMessage); message WM_SYSCOMMAND;
 
...
 
procedure TForm2.WMGetSysCommand(var Msg: TMessage);
begin
  if (Msg.wParam = SC_MINIMIZE) then
    Close 
  else
    inherited;
end;

Action:=caHide; //Скрыть

If Form2.WindowState=wsMinimize Then Form2.Hide;

procedure Tformadv.DoClose;
begin
  inherited;
  sendmessage(application.MainForm.Handle, WM_MDIDESTROY, self.Handle, 0);
  action:=cafree;
end;

var i:integer;
begin
 for i := 0 to self.MDIChildCount-1 do
  if (self.MDIChildren[i].WindowState=wsnormal) or(self.MDIChildren[i].WindowState=wsMaximized) then
   self.MDIChildren[i].Hide;
end;

raise EInvalidOperation.Create(SMDIChildNotVisible);

TMDIChild = class(TForm)
   // ...
private
   procedure WMSize(var Msg : TWMSIZE) ; message WM_Size;
   // ...
 
procedure TMDIChild.WMSize(var Msg : TWMSIZE);
begin
   if Msg.SizeType = Size_Minimized then
   begin
     ShowWindow(Handle, SW_HIDE); // И скрывай форму
     Msg.Result := 0;
   end
   else Inherited;
end;