code: public ResourceKey child(String childId) {
          return new ResourceKey(bundle, id == null ? childId : join(id, childId));
      }
docs: Derives new resource key from this key with identifier as defined below:
  <pre>
  &lt;delimiter&gt; ::= '_'
  &lt;id&gt; ::= &lt;this id&gt; &lt;delimiter&gt; &lt;child id&gt;
  </pre>
  Example:
  <pre>
  ResourceKey k1 = key("mybundle","value").child("name");
  ResourceKey k2 = key("mybundle", "value.name");
  
  k1.equals(k2) == true
  </pre>
  @param childId child identifier
  @return new key with child identifier