How to order by child nodes in ElasticSearch

I'm trying to sort some products by child nodes. I have products, for each product has child node "stock" (contains productId (join field), stockId, stockQuantity, stockOnTheWay...) there can be multiple child nodes and I need to sum product stockQuantity from all stocks and sort by them. But as I learned, elasticsearch queries for sorting is difficult task.

I've tried to search in Google for similar problems and all I got from google was to use script and factor. But I doesn't know how to work with these solutions, in query like this.
mappings:

		"product": {
			"_routing": {
				"required": true
			},
			"properties": {
				"stockAgingMax": {
					"type": "integer"
				},
				"transitDate": {
					"type": "integer"
				},
				"productId": {
					"type": "keyword"
				},
				"created": {
					"type": "date"
				},
				"lotNum": {
					"type": "keyword"
				},
				"categoryCode": {
					"type": "keyword"
				},
				"vendorName": {
					"fielddata": true,
					"type": "text"
				},
				"categoryName": {
					"type": "text"
				},
				"stocks": {
					"type": "nested",
					"properties": {
						"stockOnHand": {
							"type": "integer"
						},
						"stockQuantity": {
							"type": "integer"
						},
						"stock": {
							"type": "integer"
						},
						"stockOnTheWay": {
							"type": "integer"
						}
					}
				},
				"vendorCode": {
					"type": "keyword"
				},
				"myJoinField": {
					"eager_global_ordinals": true,
					"type": "join",
					"relations": {
						"productFullId": "stock"
					}
				},
				"productFullId": {
					"type": "keyword"
				},
				"stockAgingY": {
					"type": "double"
				},
				"name": {
					"search_analyzer": "searchTextAnalyzer",
					"fielddata": true,
					"analyzer": "nGramAnalyzer",
					"type": "text"
				},
				"id": {
					"type": "text",
					"fields": {
						"keyword": {
							"ignore_above": 256,
							"type": "keyword"
						}
					}
				}
			}
		}
	}

request:

  "from": 0,
  "query": {
    "bool": {
      "should": [
        {
          "has_child": {
            "inner_hits": {},
            "query": {
              "match_all": {}
            },
            "type": "stock"
          }
        }
      ]
    }
  },
  "size": 3
}

response:

{
	"hits": [
		{
			"_index": "catalog_test",
			"_type": "product",
			"_id": "1051230_",
			"_score": 1,
			"_routing": "1051230_",
			"_source": {
				"id": "1051230_",
				"myJoinField": "productFullId",
				"productId": 1051230,
				"lotNum": "",
				"productFullId": "1051230_"
			},
			"inner_hits": {
				"stock": {
					"hits": {
						"total": 1,
						"max_score": 1,
						"hits": [
							{
								"_index": "catalog_test",
								"_type": "product",
								"_id": "1051230__stock",
								"_score": 1,
								"_routing": "1051230_",
								"_source": {
									"stocks": [
										{
											"stockQuantity": 1,
											"stockOnTheWay": 0,
											"stockOnHand": 1,
											"stock": 1030
										},
										{
											"stockQuantity": 0,
											"stockOnTheWay": 0,
											"stockOnHand": 0,
											"stock": 1031
										}
									],
									"stockAgingY": 0,
									"stockAgingMax": 60,
									"transitDate": 0,
									"created": "2019-08-02T17:40:20.1859615+03:00",
									"id": "1051230__stock",
									"myJoinField": {
										"name": "stock",
										"parent": "1051230_"
									},
									"productId": 1051230,
									"lotNum": "",
									"productFullId": "1051230_"
								}
							}
						]
					}
				}
			}
		},
		{
			"_index": "catalog_test",
			"_type": "product",
			"_id": "1051231_",
			"_score": 1,
			"_routing": "1051231_",
			"_source": {
				"id": "1051231_",
				"myJoinField": "productFullId",
				"productId": 1051231,
				"lotNum": "",
				"productFullId": "1051231_"
			},
			"inner_hits": {
				"stock": {
					"hits": {
						"total": 1,
						"max_score": 1,
						"hits": [
							{
								"_index": "catalog_test",
								"_type": "product",
								"_id": "1051231__stock",
								"_score": 1,
								"_routing": "1051231_",
								"_source": {
									"stocks": [
										{
											"stockQuantity": 0,
											"stockOnTheWay": 119214,
											"stockOnHand": 0,
											"stock": 1030
										},
										{
											"stockQuantity": 0,
											"stockOnTheWay": 0,
											"stockOnHand": 0,
											"stock": 1031
										}
									],
									"stockAgingY": 0,
									"stockAgingMax": 0,
									"transitDate": 119214,
									"created": "2019-08-02T17:40:20.1859672+03:00",
									"id": "1051231__stock",
									"myJoinField": {
										"name": "stock",
										"parent": "1051231_"
									},
									"productId": 1051231,
									"lotNum": "",
									"productFullId": "1051231_"
								}
							}
						]
					}
				}
			}
		},
		{
			"_index": "catalog_test",
			"_type": "product",
			"_id": "1053682_",
			"_score": 1,
			"_routing": "1053682_",
			"_source": {
				"id": "1053682_",
				"myJoinField": "productFullId",
				"productId": 1053682,
				"lotNum": "",
				"productFullId": "1053682_"
			},
			"inner_hits": {
				"stock": {
					"hits": {
						"total": 1,
						"max_score": 1,
						"hits": [
							{
								"_index": "catalog_test",
								"_type": "product",
								"_id": "1053682__stock",
								"_score": 1,
								"_routing": "1053682_",
								"_source": {
									"stocks": [
										{
											"stockQuantity": 2,
											"stockOnTheWay": 0,
											"stockOnHand": 2,
											"stock": 1030
										},
										{
											"stockQuantity": 0,
											"stockOnTheWay": 0,
											"stockOnHand": 0,
											"stock": 1031
										}
									],
									"stockAgingY": 2,
									"stockAgingMax": 0,
									"transitDate": 0,
									"created": "2019-08-02T17:40:20.1859778+03:00",
									"id": "1053682__stock",
									"myJoinField": {
										"name": "stock",
										"parent": "1053682_"
									},
									"productId": 1053682,
									"lotNum": "",
									"productFullId": "1053682_"
								}
							}
						]
					}
				}
			}
		}
	]
}

in this example I wanted to product with id 1053682 be first(not 3rd) because it has value 2 in stockQuantity param. But right now I have no idea how to do that.

elasticsearch version: 6.3.2