That returns the child control's Windows, an HWND. To control that window, you'll need to send it messages either directly with Send/Post Message or with GDI calls that send the messages for you.
A parent window knows if it has children. And it can get to those children. Those DlgItem functions taket he parent window and an index to the child. But it's just another way of referencing the child window handle.
If you're indexing to the wrong child, make sure you are passing in the correct index where you're currently passing zero.
For some reason I can only use it ounce, would menus count as children? Also could you give some example code with both ways you explained I learn better visually.