And once that baby is 2 years old, try right away for another. They will say "It's too soon...". Then I close the game for about 20 minutes (I play mine on fast-mode), reopen it, put the parents together again and they will usually try. I will keep closing the game and try every 20 minutes until they make another baby.

It also helps if they're fed, showered, not exhausted and not ill before trying for the baby.