gistfile1.js

# ########################
# Create mapping for Parent / Child

curl -X POST 'http://localhost:9200/families/' -d '{
    "mappings" : {
        "family" : {
            "properties" : {
                "name" : {"type" : "string", "store" : "yes"}
            }
        },
        "member" : {
            "_parent" : {
                "type" : "family"
            }, 
            "properties" : {
                "name" : {"type" : "string", "store" : "yes"},
                "role" : {"type" : "string", "index" : "not_analyzed"}
            }
        }
    },
    "settings" : {
        "number_of_shards" : 1,
        "number_of_replicas" : 0
    }
}'

# ########################
# Index parent

curl -X PUT 'localhost:9200/families/family/1' -d '
{
    "name" : "Collins",
    "motto" : "Happy day"
}'

# ########################
# Index children

curl -X POST 'localhost:9200/families/member?parent=1' -d '
{
    "name" : "Jan",
    "role" : "dad"
}'
 
curl -X POST 'localhost:9200/families/member?parent=1' -d '
{
    "name" : "Rudy",
    "role" : "mom"
}'

# ########################
# Flush

curl -X POST 'http://localhost:9200/_flush?refresh=true'

# ########################
# Use "Has Child Filter"

curl -X GET 'http://localhost:9200/families/family/_search?pretty=true' -d '
{
    "query" : {
        "constant_score" : {
            "filter" : {
                "has_child" : {
                    "type" : "member",
                    "query" : {
                        "query_string" : {
                            "query" : "dad AND jan"
                        }
                    }
                }
            }
        }
    },
  
    "fields" : [
        "name"
    ],
 
    "explain" : true
}'

# ########################
# Use "Has Child Query"

curl -X GET 'http://localhost:9200/families/family/_search?pretty=true' -d '
{
    "query" : {
        "has_child" : {
            "type" : "member",
            "query" : {
                "query_string" : {
                    "query" : "dad AND jan"
                }
            }
        }
    },
  
    "fields" : [
        "name"
    ],
 
    "explain" : true
}'

## Explanation from **filter**

      "_explanation" : {
        "value" : 1.0,
        "description" : "ConstantScore(NotDeleted(child_filter[member/family](filtered(+_all:dad +_all:jan)->cache(_type:member)))), product of:",
        "details" : [ {
          "value" : 1.0,
          "description" : "boost"
        }, {
          "value" : 1.0,
          "description" : "queryNorm"
        } ]
      }

## Explanation from **query**

      "_explanation" : {
        "value" : 1.0,
        "description" : "ConstantScore(child_filter[member/family](filtered(+_all:dad +_all:jan)->cache(_type:member))), product of:",
        "details" : [ {
          "value" : 1.0,
          "description" : "boost"
        }, {
          "value" : 1.0,
          "description" : "queryNorm"
        } ]
      }

Here is the answer:
https://github.com/elasticsearch/elasticsearch/blob/v0.19.9/src/main/java/org/elasticsearch/index/query/HasChildQueryParser.java#L111