I can see your argument from a technical perspective. Perhaps, then, instead of a rating system, a user profile system, where you can have different users on the Roku and each user could have access to different channels. Something very simple where you can log in and say "access to channel a, b c, NOT d, e and f" for user "X". User Y would have a different set of access. Protect this with a simple PIN (so that the user needs to know the PIN before they can change the access) and your are set. Then I could easily allow my kids to browse the different channels we, as parents, approve for them.
Hope this makes sense.