My 2 year old bites

I have a two year old son and he keeps biting the little girl I babysit I've tried it all time out talking to him even spanking the last time he bite the lil girl there parents were upset I don't blame them I was even more upset and I felt so bad I even went and bought her a toy I am constantly watching them and don't leave them alone together but as soon as I turn around for a second there he goes he did it today again I grabbed him as soon as I saw his mouth open but he did able to get a lil nip it's red I thinking of telling the other parents that I can know longer watch there lil girl who is also two I hate doing this because I've been watching her since she was 3 months old but i can't be stressed every time she comes over I don't know what to do I have 4 kids this is my only handful I'm stressed out I'm anxious and out of options any ideas