corrected by leethal

<select name="product[tag_ids][]">
  <option></option>
  <%= options_from_collection_for_select(tag.children, "id", "name") %>
</select>

options_from_collection_for_select include_blank

<select name="product[tag_ids][]">
  <%= options_from_collection_for_select(tag.children, "id", "name", {:include_blank => true}) %>
</select>