Not really a horror movie, I'm giving honorable mention to Tod Browning's classic 1932 melodrama Freaks, which features genuine sideshow performers and was famously banned for decades after its initial screenings. I watched it with my 3 kids today, and it held their interest despite its now-antiquated style.
Freaks is a very simple melodrama about greed and revenge, as a scheming trapeze artist elicits the affections of Hans — a wealthy little person — with plans to murder him and inherit his fortune. When Cleo's nefarious plan is discovered, Hans' fellow "freaks" serve-up some vicious carnival justice.
Notable mainly for its famous "One of us!" scene which still resonates through pop culture 80 years later, and its chilling final minutes, Freaks has remained both controversial and a potent curiosity to this day despite having its most shocking content removed and lost prior to its premiere.
Once hard to watch due to damaged film prints and poor audio quality (which wasn't helped by the thick accents of many cast members), Freaks has received some much-needed repair in recent years and is now available in some nicely restored HD versions.