It's a tricky one, as I want the consequences of pregnancy to exist and for the game to be somewhat realistic.
What I might do is make the mother unavailable for interaction when caring for her child, similar to what happens when she's at work. The free slots for dates would be when the child is at a nursery or with their grandparents. If the mother is, or becomes, too fat to care for the child, the child would be permanently moved to live with their grandparents. When the child reaches school age, I'd probably have them permanently sent away to a boarding school.
So instead of children being characters, they'd act as a second, temporary job for the mother.
I welcome any feedback or suggestions on this.