That’s what I was thinking. They’re grown ups, let them work it out.
My suggestion is that if the parent’s dates don’t know the kids, they can stay home. No reason for them to be there if there’s a chance of drama.
If they do know the kids and want to be there, invite them, seat them far enough apart that they won’t have line of sight with each other and perhaps tell them that they’ll be asked to leave if they start anything.
Considering they started dating these people almost 5 years after the divorce, they can probably suck it up for a few hours.
I’ve had to be in the same room as my ex and the guy she cheated on me with and ultimately married. Ya know what, I found a seat at the other end of the room and managed to survive for a few hours. I don’t like it, but it’s something I have to do for my kid and it’s not stopping any time soon.
I think the kids need to talk to their parents and ask them if they can keep to themselves and remain civil for a few hours. If not, can they do it if their dates aren’t there? If they can’t do that tell them they either can’t go invite one set to the ceremony and the other to the reception.
Lastly, it wouldn’t hurt for them to have someone (preferably not afraid of some confrontation) to keep an eye on them throughout the night. As long as they stay in their corners, fine. But if either of them get roudy, drunk or they get in each other’s faces, this person can ask them to leave.
PS: It probably wouldn’t be a bad idea for the parents sans their dates, to send have a quick phone/text/face to face conversation about. It’s entirely possible they can agree to put everything aside for the day or that they simply can’t be in the same room with each other and only one/none will attend.
TL;DR. They’re grownups, tell 'em if they can’t work this out on their own, they are going to have to sit out.