Unlike many other touchstones of “tween” culture (the Olson twins, for instance), it’s easy to understand the appeal of the “Princess Diaries” franchise. Star Anne Hathaway is a charmingly gawky proxy for her often equally ill-at-ease audience, mentored by the pinnacle of charm herself, the great Julie Andrews, through an appealing fantasy world of princesses and palaces. The understandable appeal…