Well, it was Safer Internet Day yesterday, OP, so all schools up and down the country will have been referring to this. Wholly unfair to blame this because the teacher is 'new' and has no children.
With regards to her not having children this is something she has remarked about herself to another mum who when asked how she would feel if it was her child being bulled her reply was well I don’t have children so I don’t know.
Poor teacher (and child being bullied to be fair) because it is so very much an unfair question of that parent to ask of someone and even worse that you were allowed to overhear it (what if the teacher can't have children or has lost a child, for example?) It never ceases to amaze me the lack of tact some people can have.
However, this level of 'tact' does also apply to your DS also and there is no harm in mentioning it to her, if not to help her be aware of how much your DS takes information literally. Just please don't be accusatory to her or blame her for your DS's reactions, which are not her fault (or anyone's fault really).
I would keep repeating to your DS with repetitive reassuring comments throughout the time he is with you, and let him talk to you as much as he feels able to. By speaking out his fears, rather than internalising them, he will learn to be able to manage it better (and this is what your job is, that you refer to, in order to help him as a parent).