title: What is the absolutely cheapest way to select a child node in Nokogiri?
body: <p>I know that there are dozens of ways to select the first child element in Nokogiri, but which is the cheapest?</p>
  
  <p>I can't get around using <code>Node#children</code>, which sounds awfully expensive. If there are 10,000 child nodes, and I don't want to touch the 9,999 others....</p>