HUGE difference in what you did and what the OP wrote about.
1. Yours was an impromptu party, thanks to your son's waffling.

You did not invite the kids as last minute seat-fillers to a party that had been planned for a few weeks. They weren't second choice.
2. You did call the parents and check to see if such a last minute party was even workable. That was smart AND considerate.
DD has been invited to a party or two like this. A neighborhood kid will decide to have a birthday party on short notice (usually waiting to see what the weather will be like) and the mom will ask, "Can you make it tomorrow? We're having breakfast at McD's and then going to the pool." They eat Happy Meal breakfasts, play a bit, open gifts, head to the pool, have cake/cupcakes there, and it's a party! (Or you could reserve the pool for $350, but most people don't want to do that for less than 10 kids.

)
You weren't hoodwinked as bad as my aunt was years ago. My mother loved to tell this story. This was back when kids could just get on any bus at school and ride home with any other kid, no questions asked. My aunt had 7-8 kids, one right after the other, so when the bus pulled up, she was used to a herd getting off the bus. One day, the kids just kept coming, one after another. Bunches of them.....and they weren't all hers.
Turns out one of her kids was having a birthday and decided to invite lots of the kids at school to his birthday party.....The party his mother knew NOTHING about.

This was back in the early 60s, and my aunt lived in a rural area with no phone. She couldn't even call the parents to come get their kids. There was no such thing back then as running up to a grocery store and getting a ready made birthday cake. So she frantically got busy making a cake from scratch and let the kids play outside. Once the cake was done, she fed them. By that time, her husband and some of the other men were getting home from work and they were able to take the kids home.
I don't know what the other parents thought when their kids didn't get off the bus, excect that apparently word spread via older/younger siblings that they had gone home with my cousin for his birthday party. It was a different world back then......But THAT'S a real last minute party.