A very young child or baby as a character in a romance novel, particularly the ever popular "secret baby", can be used to create vital conflict between the hero and heroine or, by contrast, bind them together until they are able to overcome other obstacles to their HEA.
Create a conflict between your hero and heroine with the potential for serious consequences, even life or death. The less likely it appears that their differences can be resolved in time for the HEA (Happy Ever After) ending, the stronger your story will be.
To discover the Goal, Motivation and Conflict for your characters, ask yourself these questions: What does she/he want? Why does she/he want or need it? What prevents her/him from achieving it?
The Holy Trinity of plotting is Goal, Motivation and Conflict. Construct these three things for your characters and everything else will usually fall into place.