Honestly, if I were you, I would trust the teachers, family members and neighbours cos it's impossible that all of them are in cahoots.
I would send the helper packing. I don't need physical evidence to occur before my very eyes then I change. She is good only in front of you cos you are her employer. It's not easy to find a good help but weighing the danger of my child's life in another person who doesn't seemed to care is too pricely.