As Titans66 said, the kids are locked into their gender, personality and appearance. The underlying game systems weren't initially designed for randomisation or layering so the kids have to be locked. On the plus side, that means that the kids are all canon and can appear in CK sequels :)
As for the purpose of sending them to the Nexus, it's for people who want to go through the pregnancy events again in-game (big belly, unique naughty scene, etc.). There also might be an advantage to hiding the kids in Act Three...
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@ermllmre yep, nail on the head dood. i gotta long fucked up tale of woe i could spew on this subject.... but id rather not peek into the rabbit hole. would prefer to just keep on distracting myself with something positive, a difficult as that may be, by nature im a happy person... trying to keep it that way. even in the most trying of times.... woooo... go team...!
*sigh*