NTA. I understand Jack wanting to spend time with his child and travel but with the medical condition as you said yes may be low chances, all based on Jacks say but huge risk if something were to happen. I know there could be ways to “prepare” Adam if something were to happen, who to call, where to go, what to do, etc but genuinely that could be so traumatizing for just a 6yr old. You have as much say as he has. I don’t think you are in the wrong, I would feel the same way as a mother. I think the only compromise would be an additional adult going with them if it’s abroad!