Signs

The scene showing Twyla's response to Roberta's "MOTHERS HAVE RIGHTS TOO" sign is an interesting insight into the influence each other their mothers had over them.  Roberta emphasizing the mother's rights in a school case remind us of her god fearing mother.  A woman who would have been probably a strict, structured mother to her daughter had she not been so ill.  We also see that Roberta's opposition to busing has nothing to do with the racial implications, she's simply concerned about what she feels is right by the families. Just like when she was a child and didn't understand why her mother didn't shake Mary's hand.  She doesn't think in terms of the differences of people, just in certain moral ideals that she probably isn't even sure why she possesses.
Twyla's sign on the other had ("AND SO DO CHILDREN***") marks her soreness for the encounter with Roberta's mother when they were children.  Also, she has always viewed her own mother as childish, the way she was oblivious or uncaring to how she dressed and not able to keep composure during the church service.  She seems to be saying not only do the literal children have rights, but also the mothers who may act less than motherly still have feelings too.  They are still worth something despite what Roberta's mother (and now Roberta) may think.  The way she vindictively writes "HOW WOULD YOU KNOW?" shows us she still holds Roberta more responsible for her mothers actions than her mother, because Twyla always had to be the one looking out for and corralling her own mother - she expected others to do the same.
Does this encounter prove to you that Twyla is the black character and Roberta the white?  Or are you still unsure?  There were plenty of African Americans that protested integration because they didn't want their children going to school with people they assumed would all be bigots.