Hi Kelly,
I think you should try to talk to the young mother privately, without the other girl. I 'm a mom of 2 girls 7 & 11 and it's scary how mean the girls can be. It's too bad the teacher didn't help you. Just tell that you were talking in general and you didn't mean it personally and that you're sorry if it hurt her feelings. The other girl should mind her own business.
Your typing on this post was teeny tiny - I had to put on my glasses to read it LOL. I hope things improve at school and I worry what's in store for my girls. My daughter will be a brown belt soon in karate - so at least I know she can protect herself a little. Good luck.