I would still be keeping a close eye on your daughter's involvement with the coven's children, Jaysus, if I were you. I know you believe that it won't trickle on down to the children... but it will.
When I was your daughter's age, my mother had a spectacular falling out with my best friend from toddler age's mother. Oblivious, we carried on playing together at school and - being that this was pre-Social Media/the Internet days - I don't remember even really noticing the lack of after-school playdates, or even the fact that our mothers stood on opposing sides of the school gates, ignoring one another absolutely. What I did notice, however, was a few weeks later, the way in which my best friend suddenly started saying things like "my mum says you're spoiled/naughty/stupid!" and the way in which she effectively behaved like your C did in cutting me out of our friendship group. We were 8 or 9 years old, I had no clue what she was going on about - I wasn't the one who had everything she set eyes on handed to her... I had to wait until my birthday, or the festive season for anything. I also wasn't a naughty child, because I was too terrified to stand up for myself. I did as I was told, when I was told, how I was told (Army Brat, through and through...). Years later, my mother told me that she'd caught my best friend's mother trying to snog my totally not interested father on a night out. Years later, my then-best friend tried to worm her way back into my life, when she realised that my oldest was in the year above her oldest, at the same school we'd attended... and she'd bullied me. I told her that I knew it was all in the past, but that I couldn't be friends with someone who would treat anyone that way. But her mother's embarrassment at being caught trying to latch onto my father (she was divorced, I think, at the time, but I might be wrong about that...), and then her rage at my mother telling her precisely what she thought of her, trickled on down, like shit on a hill, to mangle an old Army saying, to me. Who was clueless/oblivious to what had happened, and bewildered by the nastiness which she saw in another child who had previously been like a sister to her.
Please, Jaysus, don't let similar happen to your daughter. It damaged me for years, made me believe that I was spoiled/naughty/stupid and therefore not worth being liked by my previously close group of friends.
And it's a really good thing that your ex is sticking up for you and your daughter. Let him. Stand your ground. Just make sure the children of the coven don't 'CLK' yours. 