code: public Config getChild(int index)
    {
      Params.range(index, 0, children.size(), "Index");
      return children.get(index);
    }
docs: Get configuration object child by index. Given index should be in range so that invoking this method on empty
  children list will throw exception.
  
  @param index child index.
  @return configuration object child at requested index.
  @throws IllegalArgumentException if <code>index</code> argument is not in range.