Bedrockgames
I post in the voice of Christopher Walken
Since slavery is a fundamental aspect of Dark Sun society as opposed to say a throwaway line for one race that could easily be excluded from any particular campaign and/or that aspect of the race easily excluded, I'd say your assessment is spot on and part of the issue. Do you sugarcoat the presentation? Because that kind of creates this cartoony, sanitized version of slavery and it's effects on the people subjected to it as well as their descendants, especially those with more modern/recent ties to it... and some/many could find that presentation of it as disrespectful or dismissive.
I wouldn't say this is necessarily sugarcoating it so much as making it age appropriate.
If the issue is the audience is a certain age (and many posters have expressed the concern over that issue---though I honestly think D&D has long targeted high school-college-age and above more than 12 or under), doing it the PG or Spartacus route can work. It might not get into the aspects of slavery that would be inappropriate for young people, but it does show how dehumanizing it is (one scene that always sticks with me is Spartacus working in the mine and having his teeth examined by, I believe, Batiatus (been a while and could have that detail wrong). That is a very PG way to demonstrate these people are being treated like animals (something about checking your teeth because it reveals how long you will be a healthy slave was impactful I thought). It isn't gritty and overt. There is one scene that was cut, which I think you can still see in certain version where they strongly allude to an impending sexual assault for one of the characters. It is actually a very well written script and quite a good movie for those who haven't scene it (certainly of its time in terms of how it presents things but it has some magnificent scenes). Also it doesn't shy away from things like Roman cruelty. It depicts the crucifixion of thousands of slaves and the lead character along the Appian way at the end of the Third Servile War. I would say you can still show it to someone under 12, it still captures the evils of slavery. Crucifixion of thousands is hardly a cartoonish depiction.
Now personally I don't think it needs to be PG. I think most groups are old enough to deal with this kind of content. But I would also say it is probably best to let individual groups handle how they want to address slavery. I would make it part of the setting, because it is a cruel world where slavery is a reality and life is cheap. How groups want to address that in play is up to them I think.
In general I think most people set this dial in their own campaign. You might have something like slavery present in a setting and the GM and players arrive at an understanding of where the line is in terms of how much reality of that gets depicted. A lot of groups would shy away from certain things even if they know those things are a likely aspect of it.