So Mach...I'm glad the split play performances worked out for you and your Ex's BF isn't invited to the birthday party, but also think you need to prep for shared events in the future and decide how you're going to handle them. Like Ginger said it's about the child, not the adults.
Fortunately you're in a situation where it wasn't an affair between ExW and her new BF while you were married. So I know it's tough to see them together and maybeyou're not ready but it's a more respectful less insulting situation than an ExW/OM who violated your marriage and broke up your family. It's very likely at some point after the divorce one or both parties start dating, so it is inevitable at some point.
Also, and I say this gently because I know you've invested a lot of your life into raising these kids and I don't mean to minimize that responsibility and relationship, but you need to consider that technically they're your ExW's biological children and your step-kids. It's the parents who have custody rights, not the grandparents, and especially not the step-grandparents. It's great they see you as a dad and call you that - you must've made a difference in their lives - but you also can't expect to (for example) go to both play performances and have your ExW to not see a play at all because she may bring a date. I say that not to hurt you but out of love and just pointing out a factor in the equation.
I know it's tough, sorry man. It'll get easier over time.
Yep, its not something that is enjoyable. Kids and I are solid, and we agree to do things on different days. This event just set me up to start prepping for later. Thats why I put it here, as I know I will get great advise.
