They've spent a lot of time talking about how Stigma's heart is inherently childish, back in chapter 17. And now in this chapter, we learn how much different people were from when they were children to when they grow up, and how cold Pepesha's mother was.
It kinda makes sense that he's using whatever special powers he has to bring some part of people's childish existence to life. Either way, some power to mess with people's hearts has got to be one of the most frightening and villainous powers around.