Even in this message, you sound very dismissive! It's not about being too broke to attend.
This party was planned several weeks ago by @Cocoadoodle's account. Perhaps people did overtime so they could afford to attend.
I think SIL was quite selfish in not checking if children were allowed, when she was specifically asked the question.
Also, I don't think there is any vitriol being directed at the SIL.
Are some posts quite heated? Yes.
Are they quite angry in tone? Yes.
Instead of defending SIL, perhaps ask yourself why!
You may be in the privileged position of being able to waste money needlessly, but not everyone is!
And the posters who are quite indignant, are probably thinking of fuel costs, hotels, meals, costumes, gifts, possible time off work, possible overtime to afford this weekend away, for what? Nothing.
And it probably wasn't just the kids who were excited - it sounds like the family are spread out, so the adults probably looked forward to meeting up for a catch up.
So perhaps take a minute to think about money wasted!
And before you say they could have gone anyway, for what? For one parent to sit in a hotel room?
Perhaps more likely, most of SIL's family decided not to throw good money after bad!
The fact that MIL, SIL's mother, was fielding irated calls, tells you how to read the room!