Having just watched my almost three year-old switch to the next episode of Doctor Who (Tom Baker – The Ribos Operation) after the first one completed on Amazon Instant Streaming (thanks for the free shows, Prime), I'm a bit worried… But I've known this day was coming since she started borrowing my iPhone so she could hit the App Store and show me which apps I should get for her next.
Still, she doesn't know any of my passwords, so all she can do is watch/download the free stuff on her own. And, watching those Doctor Who episodes is something we do together as a family (much to her mother's chagrin when I'm not around)… So she even "gets" that certain things require a parent to do, or at least approve of (in the case of the App store).
I'd say negotiate what you can, while they still treat you as their nominal superior. Also, try to be fair and consistent above all else – that's really the main key to having sensible and obedient children. When those cracks of "because I said so" start showing through, every existing rule becomes a line to be crossed just to see what will happen.