For the most part, there's something wrong with the baby if mom chews off a body part. But if she starts to chew on some of the other kits, you need to find an other female you can put the kits with, or you can bottle feed them.
First of all, you really need to move them into a small cage where everything is clean. Exposed bon is a very bad thing, and if the little guy gets an infection in the bone, you need to get him put down.
I've had a male where we had to get some of his front leg removed. He was 8 or 9 at the time.
I'm not sure I would do it again with an adult animal, but with such a young kit, it will learn to live with it.
I've had a mom and two kits, a couple of days old, to the vet, and as long as you do not stress mom will not stress and then the kits will be just fine.
Actually I've only had one female that stressed at the vets, and this was only with a single vet. For the most part, they just think it's cool to run around, but they get a bit mad when the vet handles them.
I'm not sure how well a couple of old days kit would take an operation. If the vet says there's a good chance, you might as well do it. If the risc of dying while in surgery is to great, there's really no reason for him to go trough the operation.
It all depends on the specific case.
I do know of chins that got a leg removed as very young, and they had no problem with it as adults
Good luck with it.