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

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"
   }
}

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