code: public List<Node<T>> getChildren() {
          if (this.children == null) {
              return new ArrayList<Node<T>>();
          }
          return this.children;
      }
docs: Return the children of Node<T>. The Tree<T> is represented by a single
  root Node<T> whose children are represented by a List<Node<T>>. Each of
  these Node<T> elements in the List can have children. The getChildren()
  method will return the children of a Node<T>.
  @return the children of Node<T>