OP, I've just had a thought.
And there's no diplomatic way of writing it, so I'll just say it how it is, and hope you don't take offence.
Have they got a problem with your DC? As in your "little Suzy" has been pulling their "little Jane's" pigtails and they think you know about it, and are doing nothing about it?
That would explain why one of them has taken umbrage with you. Has perhaps said to the other three, "that's Suzy's mother, you know, the mean one who's been making my little Jane cry, and the mother does nothing about it"
Then the other 3 might smile while on their own out of politeness, but when the 4th (Jane's mum) is about, they blank you, to show support for her.
Or, perhaps her Jane has been mean to your Suzy. And Suzy has told the teachers, and the teachers have had a word with Jane's mum, and Jane's mum does not take well to her wonderful daughter being called out. Again, Jane's mum presumes you know about it, and blanks you in behalf of your Suzy "telling tales" on her perfect daughter.
Any chance something like this is going on?