You can adopt a baby in the game once you have purchased the Paris castle.

You will be sent to the castle ground floor, where an action pin above the baby bouncer will appear. You will see a telephone icon there, if you tap on it, a popup will appear which says "Call for adoption?”. Pressing on the "Call" button activates a telephone call where you have the choice to get a baby with the gender of your preference.

Once the call is over, you need to wait 24 hours for the baby to arrive.