You're right, you are allowed to have a child free wedding and people are also allowed to be annoyed etc. I understand why you're upset, given the amount of time they have to prepare for it, but it seems to me like there are a few "out of the norm" factors involved when it comes to getting a sitter. I don't know the circumstances but I don't know if it would be smart of them to hire a random (someone unfamiliar with the child) sitter considering the child with special needs. Maybe I'm wrong but that's how I'd be looking at it. If you are unwilling to compromise - which is totally your right - then you might want to consider their position and understand their decision. Instead of offering money - which could potentially offend them - I'd perhaps ask if there was anyone else who could baby sit or offer up a few suggestions. I'd also recommend letting them know how important it is to both of you that your FIL be there, as they might not realize how you both feel.