gistfile1.rb

require 'nokogiri'
document = Nokogiri::XML(File.read('app/etc/local.xml'))

child_node = Nokogiri::XML::Node.new("test_child", document))
child_node.content = "foo"

document.xpath('config/global/resources/default_setup/connection')[0].add_child(child_node)

File.open('app/etc/local.xml.modified', 'w') { |f| document.write_xml_to f }

# <config>
#   <global>
#     <resources>
#       <default_setup>
#         <connection>
#           ...
#           <test_child>foo</test_child>