[Note: I'm not a behavioral expert, so take this with a grain of salt.]
Cats learn not to bite from playing and social interactions. So as they grow up, they will naturally learn that biting doesn't feel good from playing with their siblings.
Clearly though, that's not good enough for your situation, so try this:
- Whenever they are looking for a nipple, wrap your pinkie in a cloth towel and dip the tip in a bit of milk. Let the kittens chew and suckle on this.
- Whenever they bite your neck: take them off, put them on the floor, and gently tell them "no". (Cats are pretty good at picking up on human vocal intonation, so you don't need to be too harsh.)
- You want to make sure they know the right thing to suck on, otherwise you may find them chewing and sucking on everything, like Elmy described in her answer.
- Whenever they are looking for a nipple, wrap your pinkie in a cloth towel and dip the tip in a bit of milk. Let the kittens chew and suckle on this.
ThisDoing both of these will:
- Teach them what they can and can't chew on
- Help soothe them
- And protectProtect your finger
- And teach them what they can and can't chew on as they grow up
Also, (I haven't tried this, but) apparently using a toothbrush to stroke a cat's head simulates the feel of their mother's tongue and helps soothe them. Try doing so after every time you feed them with the towel. This way you can set a social hierarchy by making them see you as their mother, provide some positive reinforcement for correct behavior, and also teach them to lick themselves regularly (which is good for cleanliness).
Good luck!
Footnotes:
{1} To address the feedback I got in multiple comments: When I say "reprimand" I don't mean you should yell at your babies. Of course not! I'm operating under the assumption of sanity in the reader. Simply saying the word "No" in a monotone but not too loud voice is more than enough. Scolding kittens, or babies of any kind, is not acceptable! And I'll be happy to punch anyone who does. :)
{2} I know that the word "reprimand" caries a lot of negative connotations, but I checked its synonyms and they are all a lot worse. If anyone has any better suggestions, I'll be happy to make the change! EDIT: Made the change.