A new feature has "slipped" into upcoming 0.17 (though will be experimental during that version), nested docs. It allows to map inner objects (mostly make sense with multi instance objects) within the json document as nested, and allows to do nested queries (join queries): https://github.com/elasticsearch/elasticsearch/issues/1095.
It also has powerful faceting capabilities, especially when it comes to key/value based facets like terms_stats, histogram, and date_histogram: https://github.com/elasticsearch/elasticsearch/issues/1098.
There are several benefits of using it over the parent child support. The most important one is better performance and less memory overhead. Other include better faceting integration. The downside is that there is no way to update just a nested object.