Hi, I'm so sorry for your lost. You didn't do anything wrong. That happened to me too last year when my Cat gave birth 3 little kittens. They all stopped eating and died in just 1 day. I made a research because i was really sad, confused and worried for my adult cat too because i thought it was like a virus or something but what i found was something named Fading Kitten Syndrome and it's really common.

I'm really sorry and i hope that your adult cats are ok, please give them a lot of love because the death of the kittens can affect their moods.