:-(
So sorry you had to go through that. Hopefully the other daycare families understand. Maybe send out a little note about how the child is now gone and somehow explain it in a politically correct manner.
Again, it is so unfortunate that you were put in that position, all you were doing is looking out for the child.
I suspect you just laid the ground work for the school (and perhaps the interim daycare) to build on in the coming months. If they bring up this same topic to the mom she will find it harder and harder to deny and hopefully will eventually find the courage to face it.
Sadly this was the last thing this anxious child (or you) needed to witness :-(