help with resignation

I’ve decided to leave the NF Ive been working for since September. Essentially I’ve replaced their nanny of four years because she’s having a baby. I care for the younger of two children. The middle child (age 4) is who I have the most one on one time with. The child has had the same nanny for their entire life and because I am the person the child views as “the replacement”, I’m sort of the bad guy in the kiddos eyes. I have tried so much to make it work but I can’t keep this child safe because they fight me on everything. I’ve been punched, bit, kicked, told by the child they want to kill me and hurt me. I recognize this child is in pain and needs emotional support. I don’t know how to tell the parents I can’t do it anymore. I lost a pregnancy before starting this position and it truly breaks my heart every day to be essentially at odds with a child all the time. Is it okay to quit over a text/ email? It’s so hard to talk to the NPs without the NKs completing for attention. Please any advice is helpful.