I'm leaning heavily towards her kids being Agatha's creation. Maybe all part of her plan to manipulate Wanda.
If I recall correctly, Agatha has a significant role in the episode where Wanda was pregnant. And I know she was around a lot in the next episode where they keep rapidly aging.
Another random thought. I know this doesn't fit in with the theme of the show, but how has no one called the Avengers?! Seriously, if one Avenger has taken an entire town hostage with her superpowers, and another Avenger is walking around dead, seems like they would be the best people to deal with this situation. #ijs