@wrangler,
When I first got divorced, it was a requirement to take a Parenting class to learn how to co-parent. The mantra was "The Best Interest of the Children." The x didn't take that seriously but I did. Now that the kids are adults, we've had discussions and they reiterated how fully aware of the sacrifices I made for them to keep the peace with a very combative x. They asked me why and I've always maintained it was because their interests have to be the focal point.
This whole situation is your fault. You should have, at the beginning, contacted your x, let her know about the trip and who was invited. Instead, you've allowed a seven yr old to take charge and now you're stuck. This is what happens when you don't communicate with your x.
These four children live together. They are not strangers, and it seems the two youngest bond quite well together. Although the oldest two aren't that close, they can still get along without major conflict.
I don't agree with your x, that if one can't go, the other one won't either. That's not fair to your daughter who choose Jason to go. Having said that, it's not fair to Mark to be purposefully excluded when THREE of his siblings go on a trip without him. That just seethes resentment and anger.
I also concede the point that Life isn't fair, Simba. But what lesson is the one you want these kids to learn?
I'd suck it up and take all the kids, all five of them. It's just the right thing to do. Kids won't look at this the same way you do. They will remember this trip. Don't make it a bad memory.