I read the other question & it is hard to say if what my family does would in any way help you as I do not know what you have communicated to her and what she is willing to work with. In my home, we make family rules together (everyone decides on what is fair in a family meeting) & my children are permitted to call us on the rules we break. So for instance, my child may say to me or my husband, "Hey no yelling" if someone has gotten louder than we should. What you refer to as dragon face we call mean face & someone may be asked to "not make mean face at me". We also agree to not talking under our breath, no eye rolling, etc. (on top of other typical things like no hitting, no taking other people's things without asking, etc). Anything I ask my children to do in behavior, I am also agreeing to in my own behavior. We also wrote into the rules that when we break a rule, we need to make amends with the person we wronged. This empowers my children to speak what they see. I do not say things about my spouse to my kids in my spouse's presence as I feel that sets up an odd dynamic, so I would address the spouse. If your family were to have a "no dragon face rule" then I'd then say to my husband, "hey dad, no dragon face". It reads to me that you don't want to address her directly but I could be misunderstanding that. I am unsure in anything I have read on boundaries that talking about her in her presence is a strategy on boundary setting, so if that is the intention, that doesn't seem like a standard approach. It's been some time since I read on it (maybe 10 years?), but that isn't anything I recall. Everything I have read is about is addressing the person directly & calmly & assertively, and I think it is powerful for children to see us address such things correctly & without any hesitation. I am hoping that I misunderstood, as it seems she is speaking to the kids in your presence, when you feel she should be addressing you, so you need to address her to set the healthy boundary in such a case & redirect the energy. I also think that even at their young ages, based on seeing how my own kids are, they are capable of calling out parental missteps if missteps have been agreed to & outlined for them. I have had my own kids as young as 3 remind me of rules. I don't make it their job to remind us, but I empower them to know they can call it when they see it. We (as parents) agreed to that in the rule making meeting with the kids. We also remind one another if need be.
If you are instead wanting a way to talk to the kids about it when she is not there, again, based on what I know of boundary setting, it's worthwhile to discuss what abusive behaviors are, but the boundary still must be set with her in the moment itself & directly with her. That would be the most healthy for them to see.
I grew up with a dad that is a lot similar to the things you describe & maybe not so ironically, my spouses's mother has a lot of those same traits (likely why we both relate to one another and why we work hard in parenting to avoid it).
My mother in law is seldom around and my husband hasn't really ever called her out on any of it. I do not feel it is my job to deal with their relationship, so I just set up boundaries for myself & around my kids. Mostly with her I repeat the same statements over and over, such as "I really do not wish to speak to you about this. This isn't something I want or need your input on. I do not wish to speak to you about anything when you are this upset. We should talk later. I won't have this kind of thing around my kids." I say all of those many times. I really will not engage in an argument with her at all about anything & I never have. Usually we can get through a week together with no issues now. If I say anything it will be only one time I have to go over that. It has worked very well over many years at reducing her efforts to control or manipulate situations in our lives.
My father is around often, but things are much better for us now. It's taken a lot of work and repetition, but I do even trust him alone with my kids for short bits of time. That is not really a way you can handle a spouse, I know. I tell you that only so that it may offer hope for you on how your children will get through it. They have you. My mother was my soft place & my spouse had a great dad. As such, they gave us the tools to be resilient and try to do better with our own children, as I believe you will do this too. There is every reason to hope that yours will be okay, even if it's hard going through it. And I think you are right to be concerned. When I was very small my father was kind. It wasn't until I was old enough to start holding my own opinions or he started to think I was "taking my mom's side" that he became cruel to me and my siblings. I used to hope they would divorce so that I didn't have to live with him anymore and then I'd remember that if they did, I'd have to visit him without her protection & then I'd return to being grateful that she stayed.