Reproduction of sorting on child property (Note that you need 0.90.1)

create-index.sh

#!/bin/sh

# assume the index products doesn't exist
# otherwise uncomment the following line
# curl -XDELETE localhost:9200/products

curl -XPUT localhost:9200/products/product/1 -d'{
  "property1": "value1"
}'

curl -XPUT localhost:9200/products/product/2 -d'{
  "property1": "value2"
}'

curl -XPOST localhost:9200/products/offer/_mapping -d '{
  "book":{
    "_parent": {"type": "product"}
  }
}'

curl -XPOST localhost:9200/products/offer/1?parent=1 -d '{
   "color": "blue",
   "size": 1,
   "price": 99.4
}'

curl -XPOST localhost:9200/products/offer/2?parent=1 -d '{
   "color": "red",
   "size": 2,
   "price": 100.5
}'

curl -XPOST localhost:9200/products/offer/3?parent=2 -d '{
   "color": "blue",
   "size": 3,
   "price": 100.7
}'

result.json

{
  "took" : 1,
  "timed_out" : false,
  "_shards" : {
    "total" : 5,
    "successful" : 5,
    "failed" : 0
  },
  "hits" : {
    "total" : 2,
    "max_score" : 1.0,
    "hits" : [ {
      "_index" : "products",
      "_type" : "product",
      "_id" : "1",
      "_score" : 1.0, "_source" : {
  "property1": "value1"
}
    }, {
      "_index" : "products",
      "_type" : "product",
      "_id" : "2",
      "_score" : 1.0, "_source" : {
  "property1": "value2"
}
    } ]
  },
  "facets" : {
    "size" : {
      "_type" : "terms",
      "missing" : 0,
      "total" : 2,
      "other" : 0,
      "terms" : [ {
        "term" : 3,
        "count" : 1
      }, {
        "term" : 1,
        "count" : 1
      } ]
    }
  }
}

tests3.sh

#!/bin/sh
curl -s -XPOST 'localhost:9200/products/product,offer/_search?pretty=true' -d '{
  "query" : {
    "has_child" : {
      "type" : "offer",
      "score_mode" : "max",
      "query" : {
        "custom_score" : {
          "script" : "-doc['\''price'\''].value",
          "query": {
            "term" : {
              "color" : "blue"        
            }
          }
        }
      }
    }
  },
  "facets" : {
      "size" : {
          "terms" : {
              "field" : "size"
          },
          "global" : true,
          "facet_filter" : {
            "bool" : {
              "must" : [
                { "term" : { "color" : "blue" } }
              ]
            }
          }
      }
  }
}'

The following query should work:

curl -s -XPOST 'localhost:9200/products/product,offer/_search?pretty=true' -d '{
  "query" : {
    "has_child" : {
      "type" : "offer",
      "score_mode" : "avg",
      "query" : {
        "custom_score" : {
          "script" : "doc[\"price\"].value",
          "query": {
            "term" : {
              "color" : "blue"        
            }
          }
        }
      }
    }
  },
  "facets" : {
      "size" : {
          "terms" : {
              "field" : "size"
          },
          "global" : true,
          "facet_filter" : {
            "bool" : {
              "must" : [
                { "term" : { "color" : "blue" } }
              ]
            }
          }
      }
  }
}'