This is one of the aspects of HP which gives me major "kid's lit" vibes and clashes with some of the more complex themes

Well yes and no. They were demonized but not completey unfairly. Slytherins acted horribly to other houses and were shown to cheat and bully and yes this bad attitude was far spread in Slytherin students, not just the Death Eaters children... But of course not all of them are bad so it's unfortunate that we didn't get to see them more involved in the actual fighting or get to discover them in a better light. It would have been cool if Harry had made a friend in Slytherin, like he did with the Ravenclaw Luna...
BUT I also think that many fans tend to compensate for it by making them too sympatic and victimized. They are not entirely blameless.

There are some Slytherins who go beyond the demonized stereotype - Snape, Slughorn, arguably even Draco and Regulus Black.

No, you're forgetting that Slytherin can be dangerous but nobody treated Draco bad so whats your point?