Generally for nested forms, a blank form is placed below all other existing object forms. If you would like to change the position of the blank form(s) you can reorder the object's one-to-many association. For example you can put the blank form on top with the following snippet:

actors = movie.actors
actors.build
actors.unshift(actors.pop(1)) # won't work with Rails 4+

Because build_for_form creates new objects and appends them to the one-to-many association collection object you only have to reorder the collection objects.

Sorting with Rails 3+

= form.fields_for :children, @parent.children.sort_by(&:name) do |fieldsform|
  …