_form.html.erb

<% form_for @object do |f| %>
  <%= f.label :foo %>
  <%= f.text_field :foo %>
  <% f.fields_for :bar do |ff| %>
    <%= ff.radio_button :some_attr, 1 %>
    <%= ff.label :some_attr %>
    
    <%= ff.radio_button :some_attr, 2 %>
    <%= ff.label :some_attr %>
  <% end %>
  <%= f.submit %>
<% end %>

controller.rb

def new
  @object = Object.new
  2.times { @object.bar.build }
end

def create
  @object = Object.new(object_params)

  respond_to do |format|
    # other stuff
    end
  end
end

private
  def object_params
    params.require(:holiday_package).permit(:foo)
  end