This is if a story requires a child in it, Jophia will be that child. There are two times when a child is needed:
If Jophia is a child Rivatha took off the street, brought her to the hideout hoping Vess would let her stay, which he does.
When they mention the bad guy having a kids then Jophia will be Vess'. While Rivatha seems the most likely of the two to have a kid, Vess would have a closer connection to his own kid rather than Rivatha's. That being said while they won't be mentioned if Vess has kids so does Rivatha as she does have them before him.