My Biting Twins!!!

Okay, I need some advice/encouragement.
I have been caring for twin boys since January. They really are sweet and easy to care for except for the fact that they both bite! It got really bad with one of them for a while, and then I cracked down on him and it got better, however, it seems that within the last two weeks (just about the time a new child started) that the biting has increased and both of them are doing it now.
Nothing really triggers them it seems. They bite when they are frustrated that someone is trying to take their toy, but other times they will just bit for no reason. I have started to keep them contained more when I can't keep my full undivided attention on them, but it is honestly draining now that I have to keep my eyes on both of them. I do not trust them one little bit near any of the other children.
Also, my own son has just started to try and bite me and his sister. I've been on top of him, so hopefully I can stop it before it becomes a problem. I know my son is being influenced by the twins, but not that I can really say that to their parents.
What I'm really worried about is that I have another little one starting next week pt, and that they will go for him too.
I really don't know what to do. I feel like they are constantly going to be segregated from the other children for their biting. Nothing I'm doing really seems to "get" to them so that they understand what they are doing is wrong.
How long would you put up with this for? I don't really want to terminate them, as they are easy to care for, but it is exhausting to deal with.

I've only had one real biter in almost 10 years. When he started showing his "lovely skill" it disturbed me, (my children NEVER bit) I had never had to deal with it before. This one was easy for me though, he came as a "package deal" with an older brother (who bit him a few times..IN ANGER) the older brother was also violent toward me. Needless to say, I did not like either of these boys from the get go and they seemed to have a "total run" over their Mother, RED FLAG!!! He also pulled hair (for no reason). so I talked to Mom once, she got defensive. The young one bit one child...that child's parent called me in concern that her child had been bit at my house...I called the Biters Mom and her I could no longer take her sons due to the "safety" of the others. She wasn't pleased...oh well! I also found out I was like number 4 on the list of providers she had been through. Although "biting" is "normal" for some kids, it NEEDS to be stopped. Sorry, dont have alot of advice their but, keep in mind....always put yourself in ALL the parents shoes. I would be pretty upset if my child was being bit by another at daycare. :)