[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:

1. Whenever they bite your neck: take them off, put them on the floor, and gently verbally reprimand them. (Cats are pretty good at picking up on human vocal intention.)
2. If 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.

This will:
- Teach them what they can and can't chew on
- Help soothe them
- And protect your finger

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!