code: public static Node addChildNode(Document doc, Node parentNode, String childName, String childValue){
                  if(doc == null || parentNode == null || childName == null){
                          return null;
                  }
                  Node childNode = doc.createElement(childName);
                  if(childValue != null){
                          childNode.setTextContent(childValue);
                  }
                  parentNode.appendChild(childNode);
                  return childNode;
          }
docs: Add a child node
  @param parentNode - the parent node
  @param childName - name of child node to create
  @param childValue - value of child node
  @return the child node