I am not very fond of kings and queens. However, this is a recurrent theme in philately. But not so usual that little princes and princesses appear on them.
The following stamps were issued on 2012 as charity stamps, sold by primary school children to raise money for good causes. They feature the three daughters of king Willem-Alexander of the Netherlands (he was the crown prince at that moment). The pictures of the three girls – Amalia, Alexia and Ariane – were taken for their father himself.
The girls are now 11, 9 and 7 years old. If I were them, I would spend all my spare time in writing letters... Just in order to use these stamps!