Your best option is to document each and every event and meeting in writing, and send it to the head every time.
Read the school's behaviour and complaint policies, and follow them / expect them to be followed to the letter - so if the behaviour policy says 'the consequence for violent behaviour is X', and you know that has not happened, ask why.
In your position I would be requesting an urgent meeting with the Head on Monday - ask for it after school explicitly so that he / she can spend the day gathering information from the class teacher - in writing, with your record of every event that has happened so far enclosed / attached.
Be factual. State what happened, and its effect on your DD, not what you think about it (e.g. no opinion on parenting, but do record exactly what the parent said).
Your question is how the school meets its duty of care and safeguarding towards your DD. Not 'how are they going to deal with Bob?' It will help if you have an immediate list of requests - e.g. for Bob never to be included in your child's group, or for Bob to have a different playtime from your child - and a suggestion for escalation 'If there is another event, showing that the measures are insufficient to keep my DD safe, I expect a move to another class', then 'If the situation is not resolved after 2 weeks, I will be escalating this to X according to the complaints policy'
Document all meetings, and send minutes to the head, stating that unless you hear from him / her otherwise, you understand that they are treating it as a fair record of the meeting.
If no improvement / action, send the whole lot to the LA (or follow the school complaints procedure to the letter).