Talk to the mom so she knows that you canceling the play date isn't because you don't want the girls to be friends, but are thinking maybe they need some breaks since they seem to be going through a phase right now. She probably has the same concerns as you right now. That way, it won't create weirdness or she won't think you are moving away from the friendship.
I'm sure it's that normal girl drama that will pass soon enough. You could also talk to the teacher so she know that even thorough these girls are best friends, they are going through a weird stage so not to have them doing group assignments and such right now.