code: public void addChildTask(Task child)
     {
        child.m_parent = this;
        m_children.add(child);
        setSummary(true);
  
        if (getParentFile().getProjectConfig().getAutoOutlineLevel() == true)
        {
           child.setOutlineLevel(Integer.valueOf(NumberHelper.getInt(getOutlineLevel()) + 1));
        }
     }
docs: This method is used to associate a child task with the current
  task instance. It has been designed to
  allow the hierarchical outline structure of tasks in an MPX
  file to be updated once all of the task data has been read.
  
  @param child child task