I’ve just seen the update - 2 adults and 9 children at Disneyland Paris? Absolutely no chance. I’d say that even if it was a trip to Benidorm or Butlins. They may be capable, but Disney isn’t your average theme park holiday. There is so much going on, so many people, and so many opportunities for kids to wander off or get lost in a crowd. 9 kids of varying ages is also many needs and feelings for 2 adults to manage. One needs the toilet, one’s hungry, one’s crying because their feet hurt from all the walking, now another one needs the toilet, one wants to go on a ride that no one else wants to, one doesn’t want to go on a ride everyone else does want to… plus I can’t even begin to imagine how travel and sleeping arrangements will look. No chance would my 5yo go for that reason alone.
Your husband needs to man up a bit instead of sitting around with his head in his hands. 5yo is the child of you both, he does not get final say on this. MIL hasn’t done any of this in a way that’s even slightly considerate of the 9yo’s feelings. Okay she doesn’t have to take 9yo, but it’s just common decency to try not to rub it in their face. Telling the children first, so they can excitedly talk about it with each other, knowing that one child has been left out is not only manipulative, but cruel too. Her apologising for not mentioning to him first doesn’t mean anything.
Please, standup for your 9yo, and push back on your husband insisting that 5yo go. From your other posts it sounds as though he (subconsciously or not) is othering your daughter as your daughter. She’s an afterthought, so you need to be the one that stands up for her. Tell him you don’t want youngest going for a myriad of reasons, including the fact that you don’t feel comfortable of the adult to child ratio, nor your 5yo being abroad without you there, plus the irreparable damage this will do to 9yo’s relationship with other family members (especially their sister) and their emotions.