def get_children(self, root=None, target=None):
        """ return list of all the children of root of given tree
            :param root: treectrl item, treeitemid
            :param target: treectrl object
        """
        if root is None:
            print("Warning: TreeCtrl root/child must be given.")
            return None
        if target is None:
            print("Warning: TreeCtrl target must be given.")
            return None
        child, cookie = target.GetFirstChild(root)
        child_list = []
        while child.IsOk():
            if target.ItemHasChildren(child):
                child_list.extend(self.get_children(child, target))
            child_list.append(child)
            child, cookie = target.GetNextChild(root, cookie)
        return child_list
return list of all the children of root of given tree
            :param root: treectrl item, treeitemid
            :param target: treectrl object