"WP-Members™ is a plugin to make your WordPress® blog a membership driven site. Perfect for newsletters, premium content sites, and more! The plugin restricts selected WordPress® content to be viewable by registered site members. Unlike other registration plugins and WordPress® itself, WP-Members™ puts the registration process inline with your content (and thus your branded theme) instead of the native WP login page. WP-Members™ works "out-of-the-box" with no modifications to your theme, but it is scalable for users that want to customize the look and feel, or want to restrict only some content. It is a great tool for sites offering premium content to subscribers, and is adaptable to a variety of applications."
That's the main problem with the idea of child themes: What's with all the plugins the need editing the php-corefiles in case of theme update. I have no idea for a useful working routine in that case.