What does Erma tell her grandchildren about her own childhood? Why do you think she shares this information with them?

From the text:

Once she pulled out a framed photograph of her father and told us he was the only person who had ever loved her. She talked on and on about how much she'd suffered as an orphan at the hands of her aunts and uncles who hadn't treated her half as kindly as she was treating us.

In my opinion, Erma wants the children to feel sorry for her.... she's also trying to excuse her own treatment of the children.