Long ago I worked at Wal-mart’s tire center while going to college and to get as many hours as I could they let me work with the overnight people after we closed at 7. In theory. The problem was the overnight managers never got told about this so I would just hang out doing nothing for 3 hours every night and getting paid. This went on for 3 months until I got a better job and no one ever questioned me about it.
I’d kind of see that as a bonus. Some time to bond while on a nice vacation on someone else’s dime would be cool especially if it meant not having to deal with the ex and the new guy as much. Definitely don’t go if you’re not going to be civil with the ex and him though (which would be understandable given the circumstances). Ruining the trip for everyone else won’t gain anything other than possibly resentment from the kid.