Could you text her something like -

Hey Larla, just checking in about Saturday plans. Would love it if the golf/market plan still works, but if the boys golf doesn’t pan out I may need to pivot here for some family time for the 4 of us.

I guess it depends on how the misbehavior is impacting the event. Like if the child is out of control and young I can see why they wouldn't want the child to make a scene and ruin the event. If the child just got a D on their report card, that has nothing to do with the event that was already planned and the consequence should be about future plans, not ones already made.

I know a mom who does this all the time with her kid. Once out of at least 10 times it was my kid's birthday. Now, I generally see it as an excuse for not doing this and she's probably double booked her kid and chosen a different activity. Sadly, the kids are still friends, but we don't do anything with the family anymore. I noticed the kid showed up to get a big award in a t-shirt the other day despite an email going out asking kids to dress up. They are pretty much a main character family and everyone else is just easily dismissed depending on their whims.

I never ever would do this.

When I was young my BFFs parents would do this a lot.

I loathe the behavior. I would shred the mom over it.