Lucien, your earlier post got me thinking, and I looked at the Dutch royal jewels site and picked out the tiaras I thought would be most likely for the little princesses.
For at start: (and this is assuming they have all the tiaras at their disposal)
For Amalia, the hereditary Princess of Oranje: The Floral Motif Tiara, (her grandmother's favorite), or the Pearl Button Tiara (her grandmother's other favorite). I would also think that the tiara from the sapphire parure would be stunning on her. (or her mommy)
For Alexia, Princess of the Netherlands, Princess of Oranje-Nassau, Jonkveuw von Amsberg: The diamond and pearl Tiara, Princess Maxima's Diamond Stars Tiara,
For Ariane, Princess of the Netherlands, Princess of Oranje-Nassau, Jonkvuew von Amsberg: The Diamond Bandeau Tiara, the Emerald Tiara