I based my guess on the fact that the newest royal baby is Mia born to Zara Phillips in January 2014 which makes her "newer" than William and Kate's little 'un :-)
Thanks all. I think I'm going to go with Mamma Mia, as "Kiss Me Kate" is the answer to another question in the quiz (Did Will say this when he popped the question?). Also, the quiz setter is devious enough to build in the point made by Pooka - very clever!! Once again, thanks for your help.