How to find all children windows if they can't show themselves?

See more:

C#

How to find all children windows if the Spy++ can not show them? If I save a file via save MessageBox (Win 7) I can find only the MessageBox dialog but its childs not. I see the radio buttons and ok button and cancel button too, but I can't find them in Spy++ and the PostMessage can't click on successfully them (nothing happens).

parent_hwnd = FindWindow("MozillaDialogClass", torrent_name_Window);

SetForegroundWindow((IntPtr)parent_hwnd);

// click on radio button "save as" (download dialog - mozilla):
PostMessage(parent_hwnd, WM_LBUTTONDOWN, 496, 473);
PostMessage(parent_hwnd, WM_LBUTTONUP, 496, 473);

System.Threading.Thread.Sleep(1000);

// click on OK (download dialog - mozilla):
PostMessage(parent_hwnd, WM_LBUTTONDOWN, 708, 571);
PostMessage(parent_hwnd, WM_LBUTTONUP, 708, 571);