I have had this exact thing happen to me; one year ago. I even posted here about it on two separate threads. Here is the link to each thread:
https://www.daycare.com/forum/showthread.php?t=30304
https://www.daycare.com/forum/showthread.php?t=30469
In the end, the mom gave me two weeks notice on the very same day that I was about to write my own two week notice. On a later email I received from the mother; she basically thought I was making the whole thing up and punishing her child way too harshly; which is why she left. She was totally in denial about how serious her son's actions were.
I should not have let it go on, I should have given notice or immediate termination much sooner. It was a learning situation for me for sure.