My reception child was bullied by her teacher, the extent of which I'm only beginning to find out now (year 1, moved schools, child is suddenly thriving in a way I've never seen). I sound like "that parent", but the teacher actually got fired at Xmas time because of how she was treating the children, most of the class had problems with the teacher but there's about 4 or 5 that really copped it. Good for the school for getting rid of her but sadly for my child by then the damage had been done. She's now processing a lot of trauma. I think it's rare but these people do exist and these situations do occur.
If you are 100% confident that this is what is happening, don't be afraid to nip it in the bud and be assertive as and when you see it.
"Please remember to be kind when you address my child. I understand you are frustrated but she is still very small and is still learning"
"I don't like the way you are handling my child, do you need to step away for a minute and calm down?"
"Thank you for letting me know that XYZ happened, I will address it with Child tonight and get back to you" (that way your kid knows you have your back and isn't asked to apologise for something that they shouldn't have to, or for bad behaviour that didn't happen).
Keep a diary of every incident that your child tells you about, and of your observations. Make a record of every conversation you have with senior management, and send a follow up email with everything agreed/said and ask them to confirm that they are happy with your notes.
Sorry you are going through this x