Goldie, your ex is being the typical selfish man that made him do what he did in the first place. He will justify his actions by convincing himself that you were not a good wife and acting in such a way to give him the family life that he believes he is entitled to. It's your fault that he had an affair and had to break the family.
He has now found the woman of his dream, the one who makes him feel good and who he wants to reproduce with the family life that he was denied with you because of you. He is in fantasy land and if that fantasy could extend to you disappearing from earth and his new perfect woman could take your place and become his wonderful children's mum, life would be absolutely heaven.
So that's where he is now, fantasy land, and that's why he wants her to be there. She is most likely uncomfortable with it, but he is probably throwing all reasoning at her to convince her that by going and being involved, it makes her a wonderful step-mum to be, and even more the perfect woman he has fallen in love with. She is desperate to please him and make him believe that she is the one, she is going along with it.
The future will probably go that way. She will gradually realise that he is a selfish * and that when he is over the honeymoon stage, will start treating her like he treated you. She's walk out.
Or she will go full on with it, but if she does, hopefully, she will show to actually care, and even though it is really hurtful now, it will get easier as the years go by (for info, I would have been totally distressed if she's come to a parents evening in the first few years she came into the life of my kids. 8 years on, I appreciate that she has a significant part into their lives and her being involved -not replacing me- doesn't bother me at all. I would expect her to be at their wedding at the top table when/if that ever happens).
She will make efforts to start with, and then realise that 1/ it's pointless and will feel more assertive to tell your ex, or she will get bored in getting involved and step away anyway.
If it helps, try to look at the bigger picture than just this event. There will be more of these battles, so it's all about how you deal with them so that your children are not affected and it doesn't break you. In the scale of thing and when you'll look back in 10 years time, this open day event will be quite meaningless. It would be lovely to both be there as parents, but if it doesn't happen, it won't mean that your child will fail his education.
As said, go there for your son and keep your head high. Enjoy the evening with your child and if at some point, he goes to spend time with his dad, divert yourself to go and speak with teachers independently and if you feel very awkward, tell yourself that she probably will feel even more awkward than you.