code: public TreeElement getChild(int index)
      {
          TreeElement childNode = null;
          if (_children == null)
              return null;
          Object child = _children.get(index);
          if (child != null) {
              childNode = (TreeElement) child;
          }
          return childNode;
      }
docs: Return the child node at the given zero-relative index.
  @param index The child node index
  @return the child node