Although it makes it drawn out and does not give you the satisfaction of a response, I think the way you have done it is the best rather than confrontation. Anyone rude and self-interested enough to use friends for unpaid child care or for furthering their careers is not someone who might want to learn and grow from feedback.
You have situations where you must see these people and you know mutual people. Why invite them badmouthing you?
If confronted by either couple on why you no longer see them, just reply with something like, “Our schedules just don’t seem to be meshing these days. I hope you have a good time doing X.”
A friend of mine said, “Never explain. It just draws you into an argument.” If asked to do something he did not want to do, he would say, “I’m sorry that’s not possible.” If a rude lout persisted, he would simply repeat, “I’m sorry that’s not possible.” It leaves the other person with nowhere conversationally to go except, “Well, OK, thanks anyway” or the like.
Here’s wishing you more congenial couple friends in future.