Logstash create elasticsearch document with array as property and put new element to array

Question

I have a document with some children inside, something like:

parent : {
  "id" : 1,
  "name" : "test",
  "children" : [
     { "idchild" : 1, "name" : "c1"},
     {...}
  ]
}

I want to add the children to the already existent parents.

I have a logstash file to read all the parents. It's write the document in elasticsearch and create an empty array :

input {
    jdbc {
       jdbc_connection_string => "jdbc:postgresql://localhost:5432/test"
       ...    
    }
}

filter { ruby { code => "event.set('children',[]);" } }

output {
    elasticsearch {
        index => "test"
        user => "elastic"
        password => "elastic"
        document_type => "parent"
        document_id => "%{id}"
        hosts => "localhost:9200"
   }
}

It works fine, the documents are as expected.

In the second step i read the children, match document by parent id and update with child...

filter {
    mutate {
      add_field => { 
        "[children][child][idchild]" => "%{id}"
        "[children][child][name]" => "%{name}"
      }
   }
}

but the array children became a property and the object "child" is updated each time instead of new insert.

How can i put new child inside children? There is a way to declare idchild as id of the sub-document ?