I’m sure this must have been done before in some form, but at least I haven’t read any mommy bully-stories involving this: The basic idea is that the mom speaks a foreign language that her son/family doesn’t.
So for example: A typical housewife soccer mom takes Spanish lessons (just out of interest or maybe because she thinks it’s important for social/racial equality or something) and then goes on a family vacation with her husband and son in a Spanish speaking country. There she will flirt and talk shit about her son and husband with local men right in front of them, without them understanding what’s happening. Scheduling hook-ups, exchanging information where to meet up and fuck later on etc. Getting instructions from the men on how to translate their conversation to her son and husband and come up with a excuse to go fuck right away. You get the idea, there are many ways to go with this.
Of course this could be done with any foreign language but I feel like Spanish might be the best since it’s probably the most common second language among English speakers. Also if I’m not mistaken there are some Spanish speaking writers on this very forum. But you could also consider adding translations of the Spanish dialogue in parenthesis or something, to make it more accessible for non-Spanish-speakers. But maybe that will ruin the experience, idk, guess it depends on how you do it.
Anyway, I hope this might inspire some writer to try this concept out! I would love to read a story like that. The idea of the son (and the husband) being cut out of the conversation like that while the mom has the freedom to pretty much talk the sexiest and nastiest shit in front of them, making fun of them, receiving filthy compliments etc. sounds very promising to me.