It's not really possible to make any of Selina's Pre-Nu52 Catwoman run canon and have it make a lick of sense because of what King has done with Holly.
It'd be a lot simpler for Bruce and Selina to just have the kid in present time. And that's the main thing I'm hoping for out of this mess. Bruce deserves both of his daughters back
The likelihood of them getting stuck with a baby is so low. There's a reason why Catwoman couldn't keep her baby. An aged up kid is easier to fit. Helena is a popular character this is the chance to bring her into main continuity
Even if not that baby they could still do the unknown daughter thing. This marriage isn't going to last this could be a way to get something lasting from it
So would you want her to be 4-6 and have her hang around the BatFam Lian Harper style, or would you make her closer to Damian's age and stick her in a costume?
As discussed on the Cass thread when a few of us were talking about the theory that Cass is actually Bruce and Shiva's kid in this continuity, it'd be pretty dang silly for Bruce to have ANOTHER kid he didn't know about.
Tynion is very inconsistent when it comes to saying what Cass's age is. I'm pretty sure at one point or another he's said everything between 14 and 17. They definitely didn't act like they were close, but remember we've seen that Bruce's memories have been repeatedly messed with.
While it would be interesting to see her about lian's age her chances of survival are increased if she is older so i would have her older and stick her in a uniform. But Gotham is over run by costumed family members and I'm not sure if another one can be sustained.
It is silly but so is everyone who ended up in the Batfamily going to see the Flying Graysons and all of them leaving inspired by and in awe of Dick Grayson age 12