I know this isn’t 100% on topic but I recommend you want this TED Talk by Esther Perel about infidelity. It might give you some different perspective about cheating and how to deal with it. Take a watch and see what she has to say. She is highly regarded and a very helpful.
I do think you should go after having a conversation with your ex-wife and child about it. Talk with your child about what they hope they are getting out of it and set some expectations about you wanting to spend most of your time with them. Tell them you might be angry about the situation which has nothing to do with them. Do the same with your ex-wife and say you want to go but you might be angry at points. Say it out loud so everyone is aware before it happens.
You can’t fully disconnected from these people since you have a kid with them without hurting your child. Its going to be tough and rough but this might be a good crash course into your new relationship with your ex-wife and her new husbands. You don’t have to like them but be friendly enough in front of your kid.
Also free fancy trip. Count it has blood money for the infidelity if that helps you
Just remember that month suggestions online are for certain geographic areas. You might need to move them earlier or later. (The best rules I have seen is when nights are above 50 F in North America)