This is a two-part question.

First, which of these would be correct:
"To whom are you referring, little boy?"
or
"Who are you referring to, little boy?"

I'm leaning towards the former of the two.

The second part of the question is, are both "you" and "little boy" considered the subject of the sentence? I'm asking because I'm translating the sentence (actually "To whom are you referring, Red Devil?) into German, wherein the article and therefore the adjective form will be different depending on whether Red Devil is nominative or accusative (Roter Teufel vs. Roten Teufel). My guess is that it would be nominative, but I want to be sure. Thank you!

This is a good question. Personally, I would rather end a sentence in a preposition, if the alternative sounds too contrived.
With whom did you sit?
Whom did you sit with?
I would use the latter. I would even consider using who, because whom is fast becoming outdated in my opinion. I would probably write whom, but say who. That's my compromise.

On the subject of roter vs roten, I think this is all about the vocative case and not the acc or nom. In German (like English), there is no vocative case (used when addressing someone) really. It is simply shown by using the nom case and separating the name from the rest of the sentence with comma(s).

English: I know your brother, Dan. (talking to Dan)
German: Ich kenn deinen Bruder, Dan.
Bosnian: Ja znam vasheg brata, Dane.

(Note: In Bosnian, Dan > Dane in the voc case; i.e. when addressing Dan.)

So, my answer would be to use the nom with a comma preceeding roter.

In German, you also have nouns in apposition (I think they're called that). These copy the case of the noun they refer to. Therefore, roter would work again keeping in line with "Sie" or "du".

My translation would be (German v rusty by the way):


Wen meinen Sie, roter Teufel?

Is wen acc of wer? I forget.

Rgds

Craig

Whom would be correct. If that isnt correct my teacher is wrong

As a question, it's wrong. As a sentence (as you have it), it is feasible.

Imagine this:

Which is right who or whom?
Whom would be correct.