There seems to be an assumption that since moms are often the ones working with the kids to combat childhood bullying, they must be immune from the problem. Not at all. We may call it adult cliques, or even put it off to the social structure of the town, but what makes a bully is the same in every place and every age. One website lists bullying as "persistent unwelcome behavior, mostly using unwarranted or invalid criticism, nit-picking, fault-finding, also exclusion, isolation, being singled out and treated differently, being shouted at, humiliated, excessive monitoring, having verbal and written warnings imposed, and much more." How often has that happened in your community. Heck, in your playgroup?
Call it what it is: You won't be popular, but bullying issues need to be addressed at all levels, for the health of the whole community.
Try, if at all possible, to focus on the end result, the desired positive outcome. Ask for more discourse on the topic, again and again if you have to. Ask for the training materials that are used with the kids to be used in your group.