It’s not too late. Stick to your guns and refuse to go. Make Grandma take the kids herself while you and the wife stay home and enjoy some alone time.
All things considered, I realize you have to pick your battles where in-laws are concerned, so good luck. If you don’t put your foot down with her now though, when will you, if ever?
I can’t really think of any way to make amends to the employees on strike. Maybe go back later with some snacks and water or something, but at the end of the day, if you go to the show, you’re prioritizing your shitty mother-in-law’s lack of compassion and morals over workers making a decent wage/benefits.