I would use #2 (/parentpage/page/), since that’s the ‘official’ URL WP assigned to it, WP will find the right page faster and it may help you with your SEO.
Yes, if you change slugs it will be a broken link, you could use the Redirection plugin for that (it has the option to add a redirection from the old slug to the new slug, I believe it’s activated by default).