controller.rb

class Controller < ApplicationController
    
    # This will not increment each child
    def index
       @form = Form.new(Parent.new)
    end
    
end

form.rb

class Form 
    include ActiveModel::Model
    
    def self.model_name
        ActiveModel::Name.new(self, nil, 'Parent')
    end
    
    delegate :foo, :bar, to: :parent
    delegate :baz, :qux, to: :child
    
    def parent
        @parent ||= Parent.new
    end

    # Im hoping this will just vend a child every time it gets used by fields_for
    def child
        parent.children.build
    end
    
    def save params
        if valid?
            @parent.attributes = params[:form].slice( ... )
            @parent.save!
            # ...
            true
        else
            false
        end
    end
end

index.haml

.form
    = form_for @form do |f|
        = f.text_field :foo
        = f.text_field :bar
        
        / This child should start with a child_index of [0] 
        = f.fields_for :children, @form.child do |child|
            = child.text_field :baz
            = child.text_field :qux
        
        / ... Then Later I want to add another child, its child_index should be [1]
        
        = f.fields_for :children, @form.child do |child|
            = child.text_field :baz
            = child.text_field :qux
            
        / My problem is neither have incremented a child_index names are parent[child][field]
        / Instead of the desired parent[child][0][field]