I've been having this issue too! At first, when the message came up saying 'You can take maternity leave now', and it didn't show up on the phone, I thought maybe it would trigger once the baby was born. But now I've just come back from the hospital, with twins no less, and the notif just popped up expecting the mother to go to work in an hour! No option for maternity leave for either parent on the phone, although the option to take a vacation day does appear. But I would rather not use up vacation days that could be used elsewhere when there's supposed to be an option built into the game for new parents to take time off!
The mother in this case is in the Writing Career (Author, level 7), with 5 stars of fame and a pristine reputation.
The father in this case is in the Spy Career (Diamond Agent, Level 8), with the same 5 stars of fame and a pristine reputation.
I hope this gets fixed soon, and I'm glad that I discovered this bug on my Supersim/Rags to Riches family, rather than the Generations Challenge I was going to do it on first!
Edit: Actually, the mother, despite having four vacation days available, doesn't have the option to take a vacation day at all!!