There is not currently any functionally that will allow sub-spaces to inherit followers from their parent space. However, in cloud we do offer the news stream which lets you automatically subscribe an entire community, or a subset of users, to a selection of curated activity streams. More can be seen around News streams here What Is New About News?
If you like to see the option for sub-spaces to inherit the followers you can create an idea for it here Ideas for Jive
Hope this helps.