HWND WinChildWindowFromPoint(HWND hParent, int row, int col)

  This function determines which of a window's children occupies a
point. The top-most child which contains the point is returned.

Parameters
  hParent is the handle of the parent window.
  row and col are the 0-based screen coordinates to test.

Returns
  The handle of the child window which contains that point, NULLHWND if none.