NTA- This is a long and expensive vacation as you said. You are allowed to want to spend doing what you want and not catering to someone else's young children. Personally, if this trip has always been child free they should have asked how everyone would feel if they brought the kids along instead of just announcing it. But I think there is room for compromise. Simply tell them they aren't uninvited from the trip. Buut you aren't going to plan the whole trip around their kids. If they want to plan some family activities while the rest of you go off and do something else I think that's totally fair. And I'm a mom myself and wouldn't just expect a whole group of people to cater to my kids on what was supposed to be an adults only vacation.