Terminator 2 is nowhere close to being as dark and dystopian as TDK appears to be. I know a lot of people liked the ferry scene - I thought it was almost a deus ex machina - because the people need to show their good side (especially in a movie where people repeatedly end up dragging themselves to new lows), people will suddenly discover the good in themselves and deny Joker his triumph. Sure, Nolan directs and cuts that sequence very well, but in the end, it was still a token gesture (imo) because good must have at least some triumph. But by the end of the movie (except for that, and the faith of Gordon's boy in Batman) everything is in shambles.
Terminator 2 even had the Terminator willing to sacrifice himself for the sake of humanity - that's not really scary. TDK is scary to kids not because it shows an apocalyptic possible future (which is now almost a trope) but tries to say that society and we all stand very close to the abyss. When the Batman upsets the natural order of things, it ends up with the mobs going nuts and throwing their lot in with the psychopath Joker leading to so much mayhem and slaughter (it was on the side of the truck with the Joker in the tunnel sequence) both in terms of lives lost, property destroyed but also in terms of souls lost/scarred. That's what makes TDK scary to me.
I was revolted with what I saw. Just like I was revolted with what I saw in Seven. This is truly dark and depressing stuff. Not the finely crafted Terminator 2 - which is a great action movie. And that was what Nolan and co. aimed for and got right in spades, I think.
Kids may or may not think the Joker to be cool. There was nervous laughter (a definite audible reaction, at the very least) when Joker enters the crime conference and performs his first trick. Some kids might want to say "how cool is that" or "The Joker totally p0wned that muscle". And that is why kids perhaps should be recommended against seeing this movie. You do realize that the pencil could not possibly have penetrated the guys skull, right? It entered thru the muscle's eye and went straight into the brain matter. Can you imagine it going thru the cornea, squishing thru the eyeball... (I'll stop and not be any more graphic) But *that* was NOT cool. It was scary!
This to-me was a definite R-rated movie. It was extremely well-made and deserves to be seen and all that... But not by kids in their tens-elevens because of it's unsettling content which is sooo effective.