From the [docs](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-has-child-filter.html):

> The `has_child` filter cannot be cached in the filter cache. The `_cache` and `_cache_key` options are a no-op in this filter. Also any filter that wraps the `has_child` filter either directly or indirectly will not be cached.