<%= form_for [@child, @answer] do |f| %>
<% @parent.questions.each_with_index do |question, i|  %>
<p>(Q<%= i + 1 %>) <%= question.text %>
<%= f.hidden_field :question_id, :value => question.id, :index => "index#{i}" %>
<label><%= f.radio_button :answer, 0, :index => "index#{i}" %>A</label>
<label><%= f.radio_button :answer, 1, :index => "index#{i}" %>B</label>
<label><%= f.radio_button :answer, 2, :index => "index#{i}" %>C</label>
<label><%= f.radio_button :answer, 3, :index => "index#{i}" %>D</label>
</p>
<% end %>
<%= f.submit "SEND" %>
<% end %>

Parameters: {"utf8"=>"✓", "authenticity_token"=>"9y96VdvAbs...", "answer"=>{"index0"=>{"question_id"=>"135", "answer"=>"3"}, "index1"=>{"question_id"=>"136", "answer"=>"1"}, "index2"=>{"question_id"=>"137", "answer"=>"1"}}, "commit"=>"SEND", "child_id"=>"480"}
Unpermitted parameters: index0, index1, index2
   (0.2ms)  BEGIN
  SQL (0.2ms)  INSERT INTO `answers` (`child_id`, `created_at`, `updated_at`) VALUES (480, '2017-07-20 19:36:31', '2017-07-20 19:36:31')
   (1.0ms)  COMMIT
Unpermitted parameters: index0, index1, index2
   (0.1ms)  BEGIN
  SQL (0.2ms)  INSERT INTO `answers` (`child_id`, `created_at`, `updated_at`) VALUES (480, '2017-07-20 19:36:31', '2017-07-20 19:36:31')
   (0.3ms)  COMMIT
Unpermitted parameters: index0, index1, index2
   (0.1ms)  BEGIN
  SQL (0.3ms)  INSERT INTO `answers` (`child_id`, `created_at`, `updated_at`) VALUES (480, '2017-07-20 19:36:31', '2017-07-20 19:36:31')
   (0.3ms)  COMMIT
  Child Load (0.2ms)  SELECT  `children`.* FROM `children` WHERE `children`.`id` = 480 LIMIT 1