Figuring out what stepchildren should call their stepparents can be tricky. From what I've observed, it's most common for stepkids to call their stepparents by their first names and refer to them as "my stepmom" or "my stepdad" when using 3rd person. There's something about calling a stepmother "stepmother" that feels so mother goose fairytale land, which can be unappealing because of the reputation stepmothers have gained in such narratives.
Of course, there are exceptions when kids call their stepparents "mom" or "dad." If their biological parent is not in the picture, there's no harm in using "mom" or "dad." However, when a kid has a mom and a stepmom, them calling the stepmom "mom" can offend and disrespect the woman who gave them life. That's exactly what happened when a mother found out her ex-husband's fiancé was allowing her very young children to call her "mom." The people of Reddit have spoken, and it's clear who's the guilty party.