how often are you handling them?
and are you leaving dry food in the cage all the time, as already suggested, I have always held each kitt, and offered it my thumb(hurts less than finger) if they bite, keep thumb straight in their mouth, dont let them push or turn there head to the side, so as to bite with the side teeth, keeping thumb straight, push in and down onto back of tongue, this stops the biting straight away, and turns the tables, to them trying to get your thumb out of their mouths, do this a few times daily, they soon learn not to nip
they are not going "for you" but if you th