According to this Wikipedia article, his eldest son Carloman was made king of Italy and renamed "Pepin." This is not to be confused with another (possibly illegitimate) son named "Pepin the Hunchback." So I guess that if Jenny had said "Pepin" without adding "the Short," her answer would have stood, unless the judges decided to be really evil with a BMS.
The clue writers were obviously thinking of the musical Pippin, with that bit about "magic to do." This hardly seems fair, since the story in the musical was pretty much pure fiction, starting with the hero's name.
On another topic--I knew the TS of Seven Days in May. Highly recommended, especially if you like political thrillers.