I’m with the OP. They said no children at the start and it turns out actually there was a child, brides neice. So not really no kids. I understand how that would hurt. But I suspect they just wanted the neice for the photos which IMO is shit.
Then they changed their minds and said kids could come and be involved. So that changes the dynamic. It’s now a wedding with children.
When the kids got there they were put in a separate room and the adults supervising (who were actual original wedding guests) were not treated well. A bit like a punishment for bringing the kids.
If they didn’t want kids there they should have not had kids there (yes, brother should have had that difficult conversation with the mother), not invite one child but not the other or invite all children and treat them differently to other wedding guests.
Sounds like OP’s brother couldn’t bring himself to stand up to his mother (for kids) or his wife (against some kids) and ended up pissing everyone off.
The man who shouted at your husband is a dick though.
Your husband was right when he suggested leaving the kids at home, but then if you had done that you would have been on the end of a load of shit yourself from your mother and your brother after he had ‘graciously’ invited your kids after all.
It’s a no win situation so I would recommend just not saying anything. It can’t help.