Back to the OP, I would be especially worried about the bullies, since they bullied your dd in front of you. How brazen! It seems clear that they are not afraid of consequences from adults. I wonder what the dynamic is at school (and at home) that lets them feel so confident about their poor behavior?
I would definitely look for other schooling options for your dd, while at the same time I would put pressure on the school to solve this problem. I would email the principal with my concerns, and schedule a meeting. I would definitely want the situation written down. Ask the principal what his/her plan will be to prevent your dd from being bullied by the same children again this year. I might consider having in my hand the state law on bullying and ask how the school plans to comply with state law in this case, and point out how state law has not been complied with if that's true. I would follow up the meeting with an email with the action plan you came up with in the meeting, ending with, "please respond if this is not accurate." You want the principal to take responsibility for this situation and he or she will be well aware that a paper trail holds them responsible.
I think it's terrific that you're helping her strengthen her friendship with one girl, but the other girls need to stop the bully behavior as well. I also think it's laudable that you're trying to work with a bully-mother and hope that helps, but the school needs to stop up as well.
Good luck and keep us posted. I feel for your sweet dd!