I don't think you should say anything and please try to get over your feelings of hurt and betrayal before you see them next. You want your time with them to be pleasant, not fraught with negative emotions just under the surface. People don't like to be around angry people.
I bet it never occurred to them that her Ps shouldn't come along. They don't get the whole GP thing and wouldn't think that this was just a vacation for his side of the family. Try not to think the worst of them. Just assume they did it out of ignorance and let it go. Next time make it clear that the invitation only extends to DS, DIL and GC.
Poo is right, you didn't do anything wrong by asking just your DS, but he should have consulted with his W before committing to anything. That is between them. Next time, if you extend an invitation to you DS, you should say, "Let me know what your answer is after you talk to DIL."
DH and I are going to be struggling with something like this when we go see SD this winter. Last time we made the trip, we met them at MIL's home. This meant the entire clan was there for a week and the clan is huge (DH has 6 siblings and they all have kids). It was great seeing them, but we didn't really get any one on one time with GD or SD and SIL. This time, we want to visit them alone, but his parents live about 6 hours away from SD and if they find out we are going to be there, they will come and probably bring along some kids and grandkids. This will mean that DH and I will just be another couple of adults in the room to GD. We really don't want this, we want to spend as much time as we can with just SD and her little family. I don't know how DH is going to handle this with SD, because if MIL finds out, it will be from SD.
Before you ask, DH and MIL are not close and for that to change they would both have to unbend a bit, and neither is willing to do that. FIL simply isn't that into DH. So, not seeing them is what he wants, because MIL will create drama that he wants to avoid. She would make the trip all about her. I don't mind them. We aren't close and that is because we live so far away, but I don't dislike them and we get along when we see each other.
The two things I think you need to avoid here is carrying your anger and hurt feelings around and letting them fester. All it harms is you. And, don't make this all about you. Essentially, don't be needy or demanding. These two things easy to do, but they are bad mistakes in MIL land. By being that kind of person, you become someone who is difficult to be around, and the consequence is that no one will be around you. You will lose exactly what you want so badly. So, let the anger and hurt feelings go and enjoy what time you have with them. Make sure that they enjoy every second they are with you so that they will want to come back and will regret scheduling things the way they did.