Am I being selfish?

So, my husband is divorced- his ex-wife cheated on him however many years ago? He has 2 kids from the marriage. Every year for the kid's birthdays, his ex-wife wants to go out for the kid's birthdays- just the 4 of them. I hate this. It seems to me like she's pretending they're still a family. Why can't they celebrate their birthday separately, like they do with everything else.

And before anyone eats me alive, I have never voiced this to my husband. I am always so damn anxious during these dinners and never have tried telling him that he can't go. I'm just wondering if I am being selfish by not wanting him to (even if I do keep it to myself)?

And no, his ex-wife will not allow me being involved. She will never speak to me or go anywhere near me. She honestly acts as if him and I were the cheaters (even though we weren't)