code: public Collection<FedoraResource> getChildren(final String mixin) throws FedoraException {
          Node mixinLiteral = null;
          if ( mixin != null ) {
              mixinLiteral = NodeFactory.createLiteral(mixin);
          }
          final ExtendedIterator<Triple> it = graph.find(Node.ANY, CONTAINS.asNode(), Node.ANY);
          final Set<FedoraResource> set = new HashSet<>();
          while (it.hasNext()) {
              final Node child = it.next().getObject();
              if ( mixin == null || graph.contains(child, HAS_MIXIN_TYPE.asNode(), mixinLiteral) ) {
                  final String path = child.getURI().toString()
                          .replaceAll(repository.getRepositoryUrl(),"");
                  if ( graph.contains(child, HAS_MIXIN_TYPE.asNode(), binaryType) ) {
                      set.add( repository.getDatastream(path) );
                  } else {
                      set.add( repository.getObject(path) );
                  }
              }
          }
          return set;
      }
docs: Get the Object and Datastream nodes that are children of the current Object.
  
  @param mixin If not null, limit to results that have this mixin.