namespace Factorlabs\Bundle\UserBundle\Form\Type;

use Symfony\Component\Form\FormBuilderInterface;
use FOS\UserBundle\Form\Type\RegistrationFormType as BaseType;

class RegistrationFormType extends BaseType
{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
           // other fields
            ->add('termsAccepted', 'checkbox', array('label' => 'Accept terms'))
        ;
    }

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

<div  id="fos_user_registration_form_termsAccepted_control_group" class=" control-group"><label for="fos_user_registration_form_termsAccepted" class=" required control-label">
    Accept terms
</label><div class=" controls" ><label class="checkbox"><input type="checkbox" id="fos_user_registration_form_termsAccepted" name="fos_user_registration_form[termsAccepted]" required="required"    class="" value="1"> 
Accept terms
</label></div></div>