Nammy Proggs is "a tiny old woman whose back was so bent that she had to twist her head sideways to look up" (5.64). When Lina loses Poppy during the great blackout, Nammy Proggs scolds her, saying, "You have to keep an eye on your baby." (5.66) If Nammy Proggs is as old as she looks, then yeah, she's probably taken care of a few babies in her time.
She has an opinion about Doon's role in finding and taking care of Poppy, too: "Lucky for you Doon Harrow was around […] He's a good-hearted boy." (5.76) It seems like you have to be a real upstanding citizen to get on Nammy Proggs's good side.
When you're on her good side, though, she'll go out of her way to help you, like when the mayor's guards are looking for Doon. They ask Nammy Proggs if she's seen him, and she totally covers for him, saying he "Went off to the trash heaps just a minute ago." (15.28) Perhaps she's been around long enough to know when someone (like the mayor) is up to no good. She makes a moral choice that helps Lina and Doon escape the city.