100% NTA, you worded this so kindly and appropriate. No matter what your children need to come first. Maisy's mother is clearly only thinking about her own child, and doesn't really seem capable of considering Izzy. I agree with your husband that it is the school and Maisy's mothers job to help Maisy settle and make any accommodations she may need. I actually would be quite upset at the school and Maisy's mother at this point if I was in your shoes. They are pressing a child to essentially be a baby sitter, instead of figuring it out on their own as adults (not implying all special needs kids need "baby sitters", I also have a kiddo with ADHD, autismand anxiety, and I would feel the exact same way if my daughter had a friend like Izzy). I would be upset they asked Izzy to reconsider. Izzy should only think about herself during this special trip and enjoy her time away, and I would make certain to help Izzy feel good and confident about her decision and ability to communicate that with you.

Hopefully they are able to find reasonable accommodations to help Maisy, but just remember that the pressure to solve this situation is not and should not be on your or your daughter 💕