_daughter_fields.html.erb

<table>
  <%= f.fields_for :daughters do |daughters_form| %>
    <tr>
      <td>
        Daughter Name:
      </td>
      <td>
        <%= daughters_form.text_field :name %>
      </td>
      <td>
        <%= daughters_form.hidden_field :_destroy %>
        <%= link_to 'remove', '#', class: 'remove-fields' %>
      </td>
    </tr>
  <% end %>
</table>

_form.html.erb

<%= form_for(father) do |f| %>
  <% if father.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(father.errors.count, "error") %> prohibited this father from being saved:</h2>

      <ul>
        <% father.errors.full_messages.each do |message| %>
          <li><%= message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %>
    <%= f.text_field :name %>
  </div>
    <%= link_to_add_fields "Add a daughter", f, :daughters %>
    <br>
  <table>
    <%= f.fields_for :daughters do |daughters_form| %>
      <tr>
        <td>
          Daughter Name:
        </td>
        <td>
          <%= daughters_form.text_field :name %>
        </td>
        <td>
          <%= daughters_form.hidden_field :_destroy %>
          <%= link_to 'remove', '#', class: 'remove-fields' %>
        </td>
      </tr>
    <% end %>
  </table>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

application_helper.rb

module ApplicationHelper
  def link_to_add_fields(name, f, association)
    new_object = f.object.send(association).klass.new
    id = new_object.object_id
    fields = f.fields_for(association, new_object, child_index: id) do |builder|
      render(association.to_s.singularize + "_fields", f: builder)
    end
    link_to(name, '#', class: "add_fields", data: {id: id, fields: fields.gsub("\n", "")})
  end
end

fathers.coffee

$ ->
  $('form').on 'click', '.remove-fields', (event) ->
    $(this).prev('input[type=hidden]').val('1')
    $(this).closest('table').hide()
    event.preventDefault()
  $('form').on 'click', '.add_fields', (event) ->
    time = new Date().getTime()
    regexp = new RegExp($(this).data('id'), 'g')
    $(this).after($(this).data('fields').replace(regexp, time))
    event.preventDefault()

params when try to create a father with a daughter

<ActionController::Parameters {"utf8"=>"✓", "authenticity_token"=>"Kk59weYpfZRfHYrvZka2CSfZ5CSjOuy2a2fjZjpKhVG+ALWkuQVNo/MJnaEpMiErEwPPO/KxzJvQ7dFSHtcQLg==", "father"=>{"name"=>"Bob", "daughters_attributes"=>{"1467509946084"=>{"daughters"=>{"name"=>"Sally", "_destroy"=>""}}}}, "commit"=>"Create Father", "controller"=>"fathers", "action"=>"create"} permitted: false>
Unpermitted parameter: daughters