I think it's a "straw that may break the camel's back" scenario.
On the face of it, it's nothing much, highly plausible deniability, just a way of speaking..
But if you are stuck in that awful dynamic where your children, and maybe you and DH are second class citizens.... Where you just KNOW each phrase is picked carefully to be barbed and hurtful..Such casual differentiation REALLY gets under your skin.
All these posters sharing it's a non-event are clearly lucky enough to live without the effects of being seconded, othered, left out, not treated fairly or with natural enthusiasm and love. Little digs every fucking time. Being put in your place by an outdated, covertly rude and emotionally immature boomer, when all you try to do is be adult and pleasant and thoughtful.
It really fucking grates, it puts you in an awful controlled position.. Because throwing a bomb into the situation would upset your husband who's probably been conditioned to know his place and keep the peace, and when it involves your kids, woo-wee... It is enraging. They deserve better.
On the plus side it blatantly exposes how insecure and immature your MIL (and presumably fil?) are. I would make my own way and let your husband pick up the slack with them. Which againz is upsetting and annoying, I'm sorry OP. But your kids are definitely worth more than this not so subtle putting down from a supposed close family member. It really does a number on their self esteem, I wouldn't want to expose them to it.