trocc

I'm not sure if "the 'horror' of having a disabled child" is something being addressed as explicitly as you suggest. Yes, someone in the show has a disabled child (several actually) and treats them horribly as a result. But, it's not really because they're disabled, it's because she's the horrifically quintessential horrible mother. (Which the writers have stated comes to the forefront in the next episode.) We're finding that nothing good grows in salted earth. Which is a metaphor that can be used to symbolize both the children of Constance and her soliloquy to the Armenian in last week's episode. Also, in this last episode, we find out that the root of Tate's problem is his hatred for his mother and that she was being charged with criminal neglect...her womb and soul are deeply salted like the land the house is built on.

The creator, Ryan Murphy, includes characters with Down's syndrome as a trope in most of his shows (AHS, Nip/Tuck, Glee). Not with the intention of "showing the horror" of their affliction or the horror of "dealing" with their affliction, but stressing the humanity of the person despite the affliction. Addie (and later, to a lesser extent, Beauregard) is shown to be both intelligent and independent: standing up to a horrible mother, falling victim to her mother's failings, but able to ultimately rise above them and gets her wish not to be tied to the house. While her outside was marred, her inside was pure...upon her death she reveals that everyone else has the obverse: she's deeply frightened by Tate and chastises her mother for her horrific actions.

To Ryan Murphy, there's no horror in having a disabled child. The horror is in how the world treats them by failing to look into someone's soul.