Would your ex go if the situation was reversed? Would she be willing to “put all the bad blood behind”? Not if she had any self respect. Frankly, I think just asking you is an insult in itself. It puts the onus on you to be the “bad guy”.
Your feelings are completely valid. Your son is old enough to understand the basics. DO NOT GO. She’ll ask, but don’t bother explaining why. She lost her right to know your heart when she betrayed you.
Not having an opinion is MUCH better than sticking to an uninformed opinion with stubborn fervor. Nobody can possibly know everything, so it’s perfectly fine to take time out to research something, or decide that it’s not worth your time and forget about it completely.