mean of a value per bucket where bucket may have multiple children

gistfile1.txt

PUT test/_doc/01_01
{
  "parent_id": "01",
  "value": 10
}

PUT test/_doc/01_02
{
  "parent_id": "01",
  "value": 10
}

PUT test/_doc/01_03
{
  "parent_id": "01",
  "value": 10
}

PUT test/_doc/02_01
{
  "parent_id": "02",
  "value": 20
}

# not what we want
GET test/_search
{
  "size": 0, 
  "aggs": {
    "mean_of_three_documents": {
      "avg": {
        "field": "value"
      }
    }
  }
}

# I think this is what we want
GET test/_search
{
  "size": 0,
  "aggs": {
    "all_parent_ids": {
      "terms": {
        "field": "parent_id.keyword"
      },
      "aggs": {
        "value_per_parent": {
          "max": {
            "field": "value"
          }
        }
      }
    },
    "average_parent_value": {
      "avg_bucket": {
        "buckets_path": "all_parent_ids>value_per_parent"
      }
    }
  }
}

GET test/_search
{
  "aggs": {
    "4": {
      "avg_bucket": {
        "buckets_path": "4-bucket>4-metric"
      }
    },
    "4-bucket": {
      "terms": {
        "field": "parent_id.keyword",
        "order": {
          "_key": "desc"
        },
        "size": 100000
      },
      "aggs": {
        "4-metric": {
          "max": {
            "field": "value"
          }
        }
      }
    }
  },
  "size": 0
}