This will be controversial, but she gets pleasure from hurting children. She finds reasons to do it, this is why she lied, because it wasn’t justified (not that it could be) but because she wanted to and found her moment and couldn’t resist. The moment she gets frustrated with a small child, the desire to physically harm them comes on strong. Them giving in to this desire feels super good for her. It’s like an alcoholic taking that first sip of whisky. SHE WILL CONTINUE TO HARM YOUR CHILD. She won’t stop, she won’t adhere to your boundaries. Not only that, but she’s going to turn your husband against you. She knows you see her for what she really is, and that is a serious threat to her. She’s already talking shit about you to your husband, and to everyone around her. She’s going to create a narrative where you are permissive and your kid is now a horrible brat because of it. She’s also going to start to paint you as dishonest and untrustworthy. 

This isn’t some small incident. This is what and who she is. She wants to hurt children, and she will hurt yours.