Given that their first child only seems to have one classical French name (or at least, that is the only one known) - which hasn't been used in the family previously, I assume we may expect the same for their second child.
Boy: Alphonse or Gaétan
Girl: Juliette (although that might be too modern?) or Marguerite (but that's already her aunt's name; however, it would fit the theme of picking the name of a queen but using the French version)
Or, if they pick the same name as Louis and Marie again -as they did so for their first child-, their second daughter might be called 'Constance' as well.