I know it's been some years since this topic was discussed in its last location, but I recall this thread from time to time and, after recently finding this repost via Google, was inspired to try to find a pic of a smiling Kumari. (Hopefully it attached okay - this is my first post on this site.)
While not the full toothy smile of American standards, it is a smile. However, as previously noted, this is not necessarily a good thing as the Kumari is supposed to maintain a neutral expression. (Smiling means so many different things depending on the culture in question. I don't know about Nepal specifically, but I am aware that in other parts of Asia smiling is not necessarily so much an expression of happiness as it is a shield from shame, embarrasment, or other disturbing emotions. And in Japan it has long been customary to cover one's mouth when laughing so as not to rudely expose the teeth of an open mouth.)
I also do not know whether or not this is a Royal Kumari, or what differences that may make as far as her protocol. (The site I found it on is a travel site and didn't give any documentation with this photo - I have not posted enough here to be allowed to post the link.)
~ Mel