The answer is "we'll never know for sure," with something of a tilt towards "probably they did."

They were certainly careful to publicly appear religious. Plato's mentor was put to death for, among a few other trumped up charges, impiety. Aristotle was forced to flee Athens years later for the same crime. Socrates's defense against his own charges (recorded by Plato), is telling:

I believe in spiritual agencies, as you say and swear in the affidavit; but if I believe in divine beings, I must believe in spirits or demigods; - is not that true? Yes, that is true, for I may assume that your silence gives assent to that. Now what are spirits or demigods? are they not either gods or the sons of gods? Is that true? 

But this is just the ingenious riddle of which I was speaking: the demigods or spirits are gods, and you say first that I don't believe in gods, and then again that I do believe in gods; that is, if I believe in demigods. For if the demigods are the illegitimate sons of gods, whether by the Nymphs or by any other mothers, as is thought, that, as all men will allow, necessarily implies the existence of their parents. You might as well affirm the existence of mules, and deny that of horses and asses. Such nonsense, Meletus, could only have been intended by you as a trial of me. You have put this into the indictment because you had nothing real of which to accuse me. But no one who has a particle of understanding will ever be convinced by you that the same man can believe in divine and superhuman things, and yet not believe that there are gods and demigods and heroes. 

Socrates is arguing here that he undeniably believes in the demigod children of the gods, and that a belief in them implied a belief in the gods themselves. He was accused of not believing in the gods, so demonstrating logically that he did was an important part of his trial.

Ask evidence points to him, Plato, and Aristotle having been religious men. Given that Socrates was put to death despite his clear innocence, it is reasonable to conclude that even if Plato and Aristotle truly were irreligious, that they kept it hidden.