Draco appears to know at least as early as the summer between his first and second years at Hogwarts. I'm not sure if there's any evidence that he knew earlier than that, but it's likely. He probably wouldn't be told when he was very young as he couldn't necessarily be trusted to keep it a secret, but perhaps would have been told before starting at Hogwarts.
In Chamber of Secrets, Lucius takes his son to Borgin and Burkes to sell some "items". The implication is that there are at least some dark magic artefacts among them, as well as other items that would give the impression that he has anti-Muggle and/or pro-Voldemort views.
"You have heard, of course, that the Ministry is conducting more raids," said Mr. Malfoy, taking a roll of parchment from his inside pocket and unraveling it for Mr. Borgin to read. "I have a few — ah — items at home that might embarrass me, if the Ministry were to call..."
Harry Potter and the Chamber of Secrets, Chapter Four - At Flourish and Blotts
During the same visit to Borgin and Burkes, he also warns Draco that it's not wise to be publicly critical of Harry when he's regarded as a hero for bringing about Voldemort's fall, implying that this isn't a view that the Malfoys share.
"... everyone thinks he's so smart, wonderful Potter with his scar and his broomstick -"
"You have told me this at least a dozen times already," said Mr. Malfoy, with a quelling look at his son. "And I would remind you that it is not - prudent - to appear less than fond of Harry Potter, not when most of our kind regard him as the hero who made the Dark Lord disappear - ah, Mr. Borgin."
Even without that, Draco isn't an idiot. He's not the best student in his year at Hogwarts, but I believe it's stated in one of the books that he's one of the best. Between the rumours about Lucius that he might hear, and all of the information he'd have access to at home, I think it's likely that he'd work it out for himself even if he wasn't told by his parents.
There's a lot less information on Crabbe and Goyle, but it seems likely they would also know. Their relationship with Draco is never portrayed as one of actual friendship, and instead much more like they're Draco's servants and protectors. Draco enjoys lording his family's status and wealth over others; if he knew that Lucius was a much more important Death Eater than their fathers, I doubt he would let them forget it.