Doctor Who successfully had a very adult series spun off from it,
That's debatable. I'd agree that it had some very dark subject matter, but the portrayal and the tone was never really that dark or serious, which makes all the difference. SG-1 for example had three instances of rape in it's first season alone, but it still seems to be considered a family show. Ditto Atlantis, where we've seen severed heads and bugs forcing their way out of abdomens. I think it's the follow through on that dark subject matter that really matter, not just the fact that it's there.