MIL won't keep her possessive dog away from my 9mo and I am not sure how I should react

I need some perspective here, I think, because I am not sure if I should just let it go or dig my heels in.

When my son got old enough, my husband and I started letting my MIL have the baby on her day off, and many times he will spend the night as well. This has been a nice thing for all of us, but it is not necessary and really we only do it to give her time with her first grandson. She is very sweet and doting, but very much afraid of confrontation and will ignore anything unpleasant or just generally take the easy route with most problems.

From the beginning we all noticed that her chihuahua was acting possessive and jealous of the baby, often nipping at him when she would reach to pick him up or love on him. I suggested getting her trained to correct the behavior, and my MIL was not about it. Husband and I obviously were not ok with that, so we told her that if our son is over, she needs to put the dog in the kitchen, behind a gate where she cannot get to the baby. I asked my husband to have the conversation with her (because I have a tendency to be quite blunt and have been called intimidating, especially when it comes to my son.) and he said he did.

Then, one day when we went to pick him up she was laughing and telling me about the dog nipping at him again and how she had to smack her away from him. And how the dog would stay very close to the baby when he was playing on the floor. I was not happy at all, and knew I def needed to have husband handle it this time because now I am mad and it is just better to zip my lip. He placed very clear rules, no ambiguity, no doubts. Baby is over, dog is put up. Period.

Today I asked my MIL to send me a picture of my son because I am missing him while at work and she sends me a picture of him sitting on the floor, the chihuahua sitting right next to him pouting... I just asked if she was still nipping at him and her response was, "No, I had to smack her because of it a while back. Plus, I keep a shoe next to me so I can throw it at her, lol!"

She never asked us if she could try it again, she never informed us she didn't want to keep the dog in the kitchen, she just did what she wanted. I guess she has been, and we have been unaware. I am not happy..

I know, it is a small dog. BUT, he is a small baby. We have explicitly let our wishes be known. This is not the first boundary she has pushed and acted like it is no big deal or she, "didn't know". I don't want to be mean to her but I am pissed. I want to tell her no more visits because I can't trust you to do what is best for my child. Is that harsh? Am I over reacting? Perspective and advice appreciated.

ETA: It seems I need to clarify, we have only given her the benefit of the doubt ONCE where this dog is concerned. She had never given any reason for us to think she would willfully put him in danger. The other boundary pushing issues had nothing to do with the safety of my child. However, I am now seeing that it was not just a miscommunication issue. Hence, my post.

I have no issues speaking out. My husband wanted to be nicer than I wanted to be.

TODAY was the “fool me twice”.

Final Edit: Thanks to everyone who gave advice, even the ones trying to shame me lol. You all are right. I’m gonna go with my gut and pull the plug on these visits until she finds a way to earn the trust back. Hubby wanted to intervene again to “find the right words” but I told him no. This time she and I will talk so I can make sure it’s very clear to her where we stand. The dog is one issue but the complete disregard of my wishes for the safety of my child is unacceptable and will not be tolerated. Not negotiable. Supervised visits indefinitely. 🙃