Okay, so I was perusing the "Junior Novel" of the film in the store and they made Cinderella's child out to be a complete tool?! That did NOT sit well with me, so I came up with a theory...
First problem- the Fairy Godmother is obviously much older than Cindy, so it would be very improbable for them both to have kids around the same age. If we're going to say "screw logic" anyway...did anybody notice how detached Jane seemed from the FG? It is my suspicion that after the events of the original story, at least in this Descendants AU, the Godmother suggested a payback from Cinderella as a proverbial thanks for her help.
Since the Fairy Godmother was too old to have children of her own, she forces Cindy into giving up her firstborn. Cinderella and Charming have boy-girl twins, the girl is born first, and the FG takes her to raise as her own. Cinderella is left with that jerk Chad for a son. Years later, at Prince Ben's coronation, Jane asks her adoptive parent to put her in a dress that Mother would have worn. I don't think it's a coincidence that Jane's coronation outfit in the movie resembles Cindy's ballgown.
Possible sequel plot? [/crazy fan theories]
https://newplayexchange.org/users/99643/amanda-delalla