I used this script:
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
GUIRegisterMsg($WM_GETMINMAXINFO, "WM_GETMINMAXINFO")
$Form1 = GUICreate("Form1", 615, 438, -1, -1, BitOR($GUI_SS_DEFAULT_GUI, $WS_MAXIMIZEBOX, $WS_SIZEBOX, $WS_THICKFRAME, $WS_TABSTOP))
GUISetState(@SW_SHOW)
$Form2 = GUICreate("Form2", 200, 200, 500, 20, BitOR($WS_THICKFRAME, $WS_SYSMENU, $WS_POPUP), $WS_EX_MDICHILD, $Form1)
GUISetState(@SW_SHOW)
While 1
$nMsg = GUIGetMsg()
Switch $nMsg
Case $G