You’re damned if you do and damned if you don’t. This sounds like the kind of mom who would have written “to the parents who thought it was ok to bring Jake’s 3 younger siblings to my party” if his parents had stayed. Don’t host a party if you haven’t adequately prepared for the guests. You assumed they should stay, they assumed it was a kid’s party. Next time communicate and prepare better, but don’t play the woe is me, card.