new.html.erb

<%= simple_form_for(
  @child,
  url: case_children_path(@case)
) do |f| %>
  <section>
    <header>
      <h2>New child</h2>
    </header>

    <%= f.input :first_name %>
    <%= f.input :last_name %>
    <%= f.input :gender, as: :select, collection: %w(Male Female) %>
    <%= f.input :born_on, as: :string, label: "Date of birth" %>
    <%= f.input :ssn, label: "SSN" %>

    <div class="actions">
      <%= f.submit "Create child", class: :button %>
    </div>
  </section>
<% end %>

new_with_placeholders.html.erb

<%= simple_form_for(
  @child,
  url: case_children_path(@case)
) do |f| %>
  <section>
    <header>
      <h2>New child</h2>
    </header>

    <%= f.input :first_name, placeholder: "First Name" %>
    <%= f.input :last_name %>
    <%= f.input :gender, as: :select, collection: %w(Male Female) %>
    <%= f.input :born_on, as: :string, label: "Date of birth" %>
    <%= f.input :ssn, label: "SSN" %>

    <div class="actions">
      <%= f.submit "Create child", class: :button %>
    </div>
  </section>
<% end %>