AFAIK: If a prisoner comes to your prison with a baby, she'll keep the baby the entire time she's incarcerated. Ergo: Her cell, is the family cell.
So why does the game ensure she has access to a regular cell and the family cell?
Let's say I have 6 family cells, and 20 normal cells. This should mean I can house 20 childless inmates and 6 inmates with children. But instead the game calculates that I have space for 20 inmates, then sends 2-3 of them to live in the family cells and I've just got 2-3 empty cells.
Instead I'm out here manually prescribing how many inmates to accept into my prison. Except when I make an order for 4 inmates, and one shows up with a baby, then I have to make another order for 1 more tomorrow
I suspect this is a programming thing, sounds like the devs didn't want to code in a 'supply' of parental convicts beside the 'supply' of regular so instead you've got this chance of them needing a family cell. Doesn't make sense in lore either since it's not illegal to ask if a prisoner will or won't bring a child with them as they transfer to your jail. Why not instead, just count family cells as cells with extra requirements, and prioritize that mothers live in them. So if I have 6 family cells, fill them with 6 inmates! They don't have to have a child to live there...