Sometimes when things are planned far in advance with lots of family involved, like the trip to America (for her cousins wedding I think it was?) that can be a sort of bookend for a long term relationship. Like ‘we need to keep things outwardly ok to get through the trip’ type thing. And if they were on ok terms, but accepting they were going to separate, let’s face it a holiday with two parents is easier than one, even if other family members are on hand to help. Especially if no one’s done anything wrong, just one of those of things, and if both still get on well with each other’s families.
Sometimes, with long term breakups, it can happen so slowly like drifting and then quickly. Throw in a family wedding overseas, flights, hotels etc booked maybe a year in advance and I don’t think the holiday means they were necessarily together together at that point, just maybe that they were both able to put stuff aside and go and be a family unit with their son.
I’m sure it’s not all gwyneth style ‘conscious uncoupling’ sunshine & roses - it never is - but they are keeping it respectful & mature (as far as it appears). You really just never know.