OP,
Unfortunately by their actions they have changed the friendship, likely forever.
I can absolutely understand your hurt and upset. Very upsetting.
Bottom line is that nice kind people wouldn't dream of doing this to a third friend in a group.
They just wouldn't.
The wisest thing here to do is NOT do anything you will regret as your children will be in the same class for years to come.
Look carefully at the BIG pucture.
If you are going to be seeing them for years, believe me, the last thing you will want to do is to create a situation that makes it awkward for you and the children to be around each other.
I think they are awful, horrible behaviour.
But for you, I would advise you tread with care and don't do anything that will cause you regret in how it impacts being able to be around each other.
Primary schooling goes on for years and years.
At nearly 60 I have heard and seen it all.
I was so warned by older friends to not get overly involved with the parents of my childrens friends as it can bring drama.
Best be polite and friendly with boundaries.
In your place I would pass this off and say little.
Yes you would have liked to be included, but shit happens and these two have decided to behave like school yard meanies.
Best you know this now, because that is likely who they are.
Heads up is good.
You are so lucky that you have other friends.
Invest in them and in making new friends.
You can still see them and pass them off, but do not depend on them.
Their brand of friendship will never be the type that lasts.
Keep all your friendship options open and explore new ones.
You have every right to be hurt, but coming out the right side of this will be more important when the hurt fades.
Primary school goes on for years and friendships really can change over that time.