indexer.rb

require 'traject'
require_relative 'recusive_json_reader'
require 'traject/debug_writer'

settings do
  store "reader_class_name", "MyJsonHierarchyReader"
  store "writer_class_name", "Traject::DebugWriter"
  store "output_file", "recursive.out"
end  

to_field('renamed_id') do |rec, acc|
  acc << rec['my_id']
end

to_field 'id_plus_one', ->(rec, acc) { acc << rec['my_id'].to_i + 1}

records.ndj

{"id":1,"children":[{"id":2,"children":[{"id":3},{"id":4}]},{"id":5,"children":[{"id":6},{"id":7}]}]}
{"id":1,"children":[{"id":2,"children":[{"id":3},{"id":4}]},{"id":5,"children":[{"id":6},{"id":7}]}]}

recusive_json_reader.rb

require 'json'
class MyJsonHierarchyReader

  # @param [#each] input_stream Probably a file, but any iterator will do
  # so long as it returns a valid JSON object from #each
  def initialize(input_stream, settings)
    # ... whatever you need to do. Let's pretend it's
    # a newline-delimited JSON file, since you didn't
    # specify anything
    @json_lines = input_stream
  end

  def each(&blk)
    @json_lines.each do |jline|
      h = JSON.parse jline
      recursive_yield(h, blk)
    end
  end

  def recursive_yield(h, blk)
    doc = build_a_doc(h)
    blk.call doc
    Array(h['children']).each do |child|
      recursive_yield(child, blk)
    end
  end

  def build_a_doc(h)
    {
      "my_id" => h['id']
    }
  end
end

run.sh

traject -c indexer.rb records.ndj