It's an evening event and both princesses wear a long gown with the ribbon of the order so a tiara is highly apropriate. What is rather odd is that Caroline's three children attending the event seem to have dressed for a different occasion.
If men wear black tie women can wear dresses of various lengths or even pants. A tiara is optional for such events but only the small version of an order (the little bow) is worn as Charlene and Caroline did for the mass and celebrations at the palace. That felt odd to me. But then im not a fan of the Dutch habit of Royal ladies wearing long gowns with orders and a hat while the men wear morning dress and only the small bow of their order and have totally forgotten about the top hat that used to go with that kind of suit (think of Royal Ascot).