A thought occured to me that perhaps someone more informed can extrapolate from:
Queen Juliana had four children, all girls.
Queen Beatrix had three children, all boys. (Ditto for Princess Margriet who had four boys.)
Based on this "pattern" as minimal as it is, and seeing as the previous monarchs before Willem had all children of the same sex, I wonder if Willem and Maxima will have all girls, no matter how many children they have?
I do not know much aobut the history of the Dutch royal family to know about the monarchs of the children before Queen Juliana; perhaps someone more informed can help out and see if my little "theory" is perhaps a pattern?