To answer the op-you should not be attacked at all in your job-ever.
It is no more acceptable than a sales assistant getting slapped, kicked, spat at and called names just because the customer is an angry person.
Join a union immediately.
Speak to the teacher and tell her that from now on you will not be left alone with this child.
What on earth are you going to do when she accuses you of some grave incident towards her? Do you seriously think that the head will support you. Do you think her parents will support you?
She needs a behavioural plan will clear, precise, simple,consistant rules.
Eg if you get through morning work without trying to hurt another human or calling anyone a name then you will get a sticker on your chart. make the chart clearly visable to her at all times. So many stickers earn x reward. However if you try and (insert action that you are trying to prevent), then a cross/unhappy face/whatever will be added onto your chart.
You will have to work with the teacher on the rewards/consequences system.
At the end of each session show the child their progress.
Reiterate the reward on offer/consequence which will happen due to their behaviour.
Always use language which clearly shows the child that they are responsible for their own behaviour ie by doing x they are choosing to forfeit y.
The parents of this child need to be shown the behavioural policy.
Ideally they should follow this through at home so that the child is under no illusion what behaviour is expected at all times-throughout life.
It does sound as if the smt is failing miserably.
Meetings should be held with parents and they should be informed if their child has hit anyone , including you, and a plan in force to stop this unacceptable behaviuor.
The parents have to know that this is unacceptable.
You can refuse to restrain a child, what would the head do then?
As for how the head deals with the child-totally wrong from what you have described. Sending out mixed messages.
What is happening is this.
The child has clearly not been shown the appropriate way in which to deal with feelings. These are feelings which everyone gets, the difference is that we all must learn to recognise and control these feelings.
When she does something unacceptable you try nad instil a consequence eg sent to the head.
The head then reacts by REWARDING the child for acting inappropriately!!!!!!!
Absolutely crazy.
I could go on.
First thing join a union.
Second thing document everything.
Third tell the teacher you are pissed off and for your own safety will not deal single handedly with her if it might involve risk to you, this involves risk of accusations.
Oh and another thing you say that other members of staff are reluctant to help with team teach-I can fully understand why.