OP, I've read the thread including your later reply and have picked up the implications within it. You're not overreacting at all. They are hugely, hugely inconsiderate guests. We adopted my child 2 years ago so we were old before we became parents and that is absolutely no excuse for their behaviour, which has nothing to do with parenthood or anything else.
They should be offering to take you to eat, quite frankly, let alone being 'surprised' you've suggested the mess they've created by not warning you in advance of their dietary needs change.
For context: I am a keen host and lots of friends and family live away, and I also grew up in Cornwall and most summers when I was a child, we'd have people coming to stay with us on the basis you say - saving money on hotels, basically! My mum is a keen cook and hostess (where I get it from) so generally, like you, she enjoyed the visitors and even if she couldn't take any or much time off work, would still cater for them just a you have. Universally the visitors would a) turn up with food and/or drink and b) take us out for dinner at least once. Memorably one set of visitors, the wife was coeliac in 1988, a time when you could barely get a diagnosis let alone buy gluten free stuff. My mum and I catered for her, even cooking gluten free Cornish pasties (a living nightmare with the limited gluten free flour available at the time). Neither of us would've been happy doing so had they sprung the ceoliac diagnosis on us at the last minute!
You are not unreasonable, they are. I can see you want to maintain the relationship for your daughter's sake in the future and I'm really sorry that seems to be so important due to your circumstances. So - bite your lip as you are but do not at any point think you're being unreasonable! they are CFs!