Term aggregation on ElasticSearch join - elasticsearch

Question

I would like to perform an aggregation on a join relation using ElasticSearch 7.7.
I need to know how many children I have for each parent.
The only way that I found to solve my issue is to use script inside term aggregation, but my concern is about performance.
/my_index/_search
{
"size": 0,
"aggs": {
"total": {
"terms": {
"script": {
"lang": "painless",
"source": "params['_source']['my_join']['parent']"
}
}
},
"max_total": {
"max_bucket": {
"buckets_path": "total>_count"
}
}
}
}
Someone knows a more fast way to execute this aggregation avoiding the script?
If the join field wasn't a parent/child I could replace the term aggregation with:
"terms": { "field": "my_field" }
To give more context I add some information about mapping:
I'm using Elastic 7.7.
I also attach a mapping with some sample documents:
{
"mappings": {
"properties": {
"my_join": {
"relations": {
"other": "doc"
},
"type": "join"
},
"reader": {
"type": "keyword"
},
"name": {
"type": "text"
},
"content": {
"type": "text"
}
}
}
}
PUT example/_doc/1
{
"reader": [
"A",
"B"
],
"my_join": {
"name": "other"
}
}
PUT example/_doc/2
{
"reader": [
"A",
"B"
],
"my_join": {
"name": "other"
}
}
PUT example/_doc/3
{
"content": "abc",
"my_join": {
"name": "doc",
"parent": 1
}
}
PUT example/_doc/4
{
"content": "def",
"my_join": {
"name": "doc"
"parent": 2
}
}
PUT example/_doc/5
{
"content": "def",
"acl_join": {
"name": "doc"
"parent": 1
}
}