How do I find position of a Win32 control/window relative to its parent window?

The solution is using the combined power of GetWindowRect() and MapWindowPoints().

GetWindowRect() retrieves the coordinates of a window relative to the entire screen area you see on your monitor. We need to convert these absolute coordinates into relative coordinates of our main window area. The MapWindowPoints() transforms the coordinates given relative to one window into relative to another. So we need a “handle” of the screen area and the handle of the parent window of the control which we are trying to find coordinates of. The screen are is a “window” in Windows terminology and it is called “Desktop”. We can access the handle of Desktop by the constant HWND_DESKTOP defined in WinUser.h (including Windows.h is enough). And we can get the handle of our parent window simply by calling the Win32 function GetParent(). Now we have all the parameters required to call the MapWindowPoints() function.

RECT YourClass::GetLocalCoordinates(HWND hWnd) const
{
    RECT Rect;
    GetWindowRect(hWnd, &Rect);
    MapWindowPoints(HWND_DESKTOP, GetParent(hWnd), (LPPOINT) &Rect, 2);
    return Rect;
}

MapWindowPoints() is defined as:

int MapWindowPoints(
  _In_     HWND hWndFrom,
  _In_     HWND hWndTo,
  _Inout_  LPPOINT lpPoints,
  _In_     UINT cPoints
);

MapWindowPoints() transform the coordinates relatively from hWndFrom to hWndTo. In our case, we do the transformation from Desktop (HWND_DESKTOP) to our parent window (GetParent(hWnd)). Therefore, the resulting RECT structure holds the relative coordinates of our child window (hWnd) relative to its parent window.