function returns the root ID, the parent window ID,
a pointer to the list of children windows
(NULL when there are no children),
and the number of children in the list for the specified window.
The children are listed in current stacking order, from bottom-most
(first) to top-most (last).
returns zero if it fails and nonzero if it succeeds.
To free a non-NULL children list when it is no longer needed, use