them telling you much later after you confirmed it was a no kids thing.

That’s a different situation than what you presented in the original post and I would argue based on your annoyance in this post that’s really what is driving you being irritated not that it was a “no kids” thing overall.

All that being said still you could just decline after saying “yes” before, it should been seen as totally acceptable by the bride and groom since they didn’t do you the solid of letting you know from the jump.