It's probably a cousin's child or whatever but the kid and indeed her mother or father might be a bit of information Cassie kept hidden from "her" readers, much like the location of the One Ring . . . that's right, I know she knows where it is . . .
Anyway by what stretch of the imagination (of the Animorphs that is) would it be impossible for a Controller to find one of their books? The less factual details they share about themselves the better so if Cassie claims to be an only child whilst having an older sibling who's moved away any search of "only children" the Yeerks do would skip over her. That it's mentioned in passing even supports the theory that it's a secret and someone just screwed up their narrative and forgot to leave that particular comment out.
This couldn't be done with Jake since Tom was a controller, and as for Rachel, well the books and TV series seem to disagree about just how many sisters she has (two in the books but I only ever remember one in the show) for all we know she might have had more sisters, or a few brothers, some of them might even have lived with her father.
As for Tobias, well I'd buy that he's an only child, and Marco . . . well even though his mother was only missing for two years it's not clear how long before that Visser One had control of her, I doubt Visser One was looking to have more children.
So while it's definately either a KASU or just some cousin's child, it's more fun to pretend Cassie was being secretive.
