Sorry to say this but, I think it’s just an error translation. I’ve read the same chapter (in the Indonesia edition) and sango instead said “kohaku please take care of the kids,”. I’ve also read a fan discussion that yes, it’s a translation error. So, I guess their child’s name is never revealed then (T-T)
(Translation: Kohaku please take care of the kids)
(P.s: sorry if this ruined your day)