Add a child to current workitem

        :param child_id: the child workitem id/number
            (integer or equivalent string)

def addChild(self, child_id):
        """Add a child to current workitem

        :param child_id: the child workitem id/number
            (integer or equivalent string)
        """

        self.log.debug("Try to add a child <Workitem %s> to current "
                       "<Workitem %s>",
                       child_id,
                       self)
        self._addChildren([child_id])
        self.log.info("Successfully add a child <Workitem %s> to current "
                      "<Workitem %s>",
                      child_id,
                      self)