Neither Mildred nor the author of the book believe that Benrhard knew about it. Or perhaps he didn't want to know of course.
The interview on Knevel & van den Brink:
Player omroep.nl
Apparently it was Annejet van der Zijl who first traced down Mildred. She wrote a very succesful biography of the prince. She gave the info. to Marc as she said that this 'was not her style'.
Interestingly enough Marc van der Linden claims that prince Bernhard himself was the one who made sure that the press got the first picture of the prince and Alexia, as he thoguht it was time that people knew....
Van der Linden was too defensive IMHO. Basically he says that the lack of sources is the readers problem, not his...