Strange that Queen Beatrix takes Princess Máxima with her, while Crown Princess Mary or Princess Letzia will not attend. Princess Máxima is a mom as well, of three kids. Apparently the 'kids-problem' which Mary and Letizia have, is no problems for Máxima.
Princess Maxima has already taken her official duties, while Pincess Letizia and May don't. So, in Princess Letizia case, it is difficult, beacause People in Spain could be upset due to She could go to parties but Official duties don't.
Yellow and Serious press will critizes her. That is the reason I know Princess Letizia won't attend this event.
And It wouldn't be a problem for Pincess Letizia to go to this events, but there are differents circunstances in each case.