You have to understand what C.S. Lewis was referring to.
He wasn't referring to a good father using the authority God gave him to form morals in his children -- he was referring more to those who force adults to act in a certain way -- like in many science fiction/dystopia novels.
If you've read 1984 or Brave New World, you'd know what he was referring to.
Today, we'll probably have something like this soon, under the premise of "helping the environment":
http://www.cathinfo.com/index.php/Green-police-superbowl-commercialThey'll force us to only have 2 children, etc. all because the liberals think it's the moral thing to do -- and they'll have the power to back it up. And the men & women that will come to your door to take your kids away (because you refused to put them in public school) will have NO QUALMS OF CONSCIENCE because, after all, you're a domestic terrorist and a quasi-cult member.
That's the tyranny C.S. Lewis was referring to.