protected function addChildrenField($form)
    {
        $form->add(
            'children',
            CollectionType::class,
            [
                'entry_type'   => self::class,
                'required'     => false,
                'allow_add'    => true,
                'allow_delete' => true,
                'prototype'    => true,
                'by_reference' => false,
            ]
        );
    }