In the British pantomime 'Principal Boy' has always referred to the main male role e.g. Jack in Jack and the Beanstalk, Aladdin, Peter Pan etc. It has traditionally been played by a female, especially one with fine athletic legs.
At an early date producers found that they needed to provide material for 'all the family', not only for the kids but mums and dads too. One way of attracting the dads in to the theatre, was to have a fine pair of female legs on show throughout the performance. Hence 'the Principal Boy'. (For those who do not know anything about Pantomime, jokes are also contrived so that the same jokes can be understood on different levels, typically double entendre for the adults, and slapstick for the kids.)
For some reason the role of Principal Boy is in decline (something to do with feminism, no doubt). So if Dick Whittington is now played by a male actor, what do we call him. Is he any longer a 'Principal Boy'?