fix:update export header

lib/tallty_import_export/export.rb

@@ -211,7 +211,9 @@ module TalltyImportExport
           get_full_key(_header).to_s.in?(headers_hash.keys)
         end.map do |_header|
           _header = _header.with_indifferent_access
-          _header.merge(headers_hash[get_full_key(_header)].delete_if { |_k, v| v.blank? })
+          merge_header = _header.merge(headers_hash[get_full_key(_header)].delete_if { |_k, v| v.blank? })
+          merge_header[:children] = _header[:children] if _header[:children].present?
+          merge_header
         end
       else
         @headers = export_headers(**options.symbolize_keys)