It sounds like you have no problem reading them age-appropriate pop fiction, but you're just looking for something more advanced as a supplement, as a way to help them grow, correct?
As someone who equates reading aloud to performance art, I'd be looking for resources that can be read in an entertaining way. So I would probably reject Kurt Vonnegut, whose tone seems rather flat and dry, and look for someone with a bit more dramatic flair, like Mark Twain (although his non-fiction actually reads better than his fiction). Others I might suggest are age-appropriate selections from Canterbury Tales, and anything from mythology that can be delivered in an entertaining tone (so not Ulysses, for instance).
I'd jump on the earlier Poe suggestion, because there's tons of performance art in there. Of course, assuming you're reading before bedtime, you have to decide if that's the kind of thing you want to read out to your children just then.
