PrototypeCollectionType is a new type I made to easily, generically, manage collection form with prototypes (add + remove), in a nicer way (IMO) than in the Symfony cookbook.

crazy_cat_lady_edit.html.twig

{% extends 'TentacodeLolcatBundle::layout.html.twig' %}

{% form_theme form 'TentacodeLolcatBundle:Form:form_theme.html.twig' %}

{% block content %}
    {{ form(form) }} 
{% endblock %}

{% block javascripts %}
    {{ parent() }}
    <script src="{{ asset('bundles/tentacodelolcat/js/prototype_collection.js') }}"></script>
{% endblock %}

CrazyCatLadyType.php

<?php

// src/Tentacode/LolcatBundle/Form/CrazyCatLadyType.php

namespace Tentacode\LolcatBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class CrazyCatLadyType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('lonelynessIndex')
            ->add('numberOfCatToys')
            ->add('cats', 'prototype_collection', [
                'type' => new CatType(),
                'label' => 'How many cats is too much cats ?',
                'add_label' => "Add another cat like she does not have enought already",
            ])
            ->add('save', 'submit')
        ;
    }

    public function getName()
    {
        return 'crazy_cat_lady';
    }
}

form_theme.html.twig

{# src/Tentacode/LolcatBundle/Resources/views/Form/form_theme.html.twig #}

{% macro prototype_child_widget(child, index, disabled = false) %}
    <div class="prototype-collection-child" data-index="{{ index }}">
        {{ form_widget(child) }}

        {% if not disabled %}
            <a class="prototype-collection-delete" href="#">delete</a>
        {% endif %}
    </div>
{% endmacro %}

{% block prototype_collection_row %}
    <div class="prototype-collection" id="{{ form.vars.id }}">
        {% if label is not sameas(false) %}
            <h5>{{ label }}</h5>
        {% endif %}

        {% if not disabled %}
            <script class="prototype" type="text/template">
                {{ _self.prototype_child_widget(prototype, '__name__') }}
            </script>
        {% endif %}

        {% for childIndex, child in form %}
            {{ _self.prototype_child_widget(child, childIndex, disabled) }}
        {% endfor %}

        {% if not disabled %}
            <a class="prototype-collection-add" href="#">{{ add_label }}</a>
        {% endif %}
    </div>
{% endblock %}

protorype_collection.js

// sorry ! jQuery inside !
// src/Tentacode/LolcatBundle/Resources/public/js/prototype_collection.js

$(document).ready(function(){
    //removing an element
    $('body').on('click', '.prototype-collection-delete', function(e){
        e.preventDefault();

        $(this).parents('.prototype-collection-child').remove();
    })

    //adding a new element
    $('body').on('click', '.prototype-collection-add', function(e){
        e.preventDefault();

        var $collection = $(this).parents('.prototype-collection');
        var prototypeHtml = $collection.find('.prototype').html();
        prototypeHtml = prototypeHtml.replace(/__name__/g, getNextIndex($collection));

        $(this).before(prototypeHtml);
    })
})

// ensure having a correct index, just using length is not sufficient
function getNextIndex($collection)
{
    var lastIndex = -1;
    var items = $collection.find('.prototype-collection-child');

    $(items).each(function(i, item) {
        var itemIndex = $(item).attr('data-index');
        if (itemIndex > lastIndex) {
            lastIndex = parseInt(itemIndex);
        }
    });

    return lastIndex + 1;
}

PrototypeCollectionType.php

<?php

// src/Tentacode/LolcatBundle/Form/PrototypeCollectionType.php
namespace Tentacode\LolcatBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class PrototypeCollectionType extends AbstractType
{
    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        $view->vars['add_label'] = $options['add_label'];
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'add_label' => 'Add an item',
            'allow_add' => true,
            'allow_delete' => true,
            'by_reference' => false,
            'cascade_validation' => true
        ));
    }

    public function getName()
    {
        return 'prototype_collection';
    }

    public function getParent()
    {
        return 'collection';
    }
}

services.yml

# src/Tentacode/LolcatBundle/Resources/config/services.yml
services:
    tentacode.form.prototype_collection_type:
        class: Tentacode\LolcatBundle\Form\PrototypeCollectionType
        tags:
            - { name: form.type, alias: prototype_collection }