Generally speaking I try not to call people on their rudeness, or put them on the spot. But in the situation you describe here, I would've been strongly tempted to be forward and say, "The kids have all talked about getting together for a playdate, and I know my son would LOVE TO PLAY WITH THEM today. Do you think he could join them?"
What the heck, if they are going to so brazenly talk about something like that without inviting your kid, PUT THEM ON THE SPOT. Let them hem and haw and explain why it might violate a fire code to have an extra kid in their house or yard! I would love to hear the explanation as to why this would be *impossible.*
Do that a few times, and they'll either learn to be nicer, or they'll learn to be more discreet.
Win/win.
So sorry you had to experience that!