She needs to go to her GP and explain she's being regularly injured at work and is scared to go into school and it’s making her unwell. They will sign her off. She needs to do this asap. Ideally tomorrow, or Tuesday if she needs to go in and collect anything important. Does she have keys? Can you go with her tonight any collect her things? She can sign herself off for the first few days if needed.
She also needs to go to her union for advice and to log this. They will help. This is not okay. It's not her fault and she does not have to stay in a workplace where she is being attacked. Of the child is aged over 10, they are criminally responsible and she has the right to report assault to the police (although SLT will tell her this is unnecessary) and get it logged.
Please advise her to gather evidence of everything that has happened and all the times she has asked for and been denied help. Take photos or screen shots of any logs and emails. Any written evidence. Also photograph any injuries. Forward emails to a personal email address.
Everything else is fixable in time but her workplace is not currently a safe place and SLT are not supporting her or ensuring she has a safe work environment.
I've been through this myself and had to demand the child was not taught in my classroom (also a tiny school). I stated that I would not be teaching again until I felt that I and the other children could be kept safe.
I was lucky that I was much more experienced and my SLT were proactive and did the right thing. Our very experienced headteacher removed the child from my class immediately. Sadly, it does not sound like the SLT in your neice's school will be as helpful.
Do not let her be persuaded to put up with this "for the children's sakes". Teachers are replaceable and children are resilient. Her health and safety need to come first.