Is there a word in Dothraki for a princess? If there isn't, would you base it on a diminutive of khaleesi or a female diminutive of khal? Or would that work out to be the same thing with how you set up the grammar of the language?
The simplest answer is no. Another answer is khalakki. Fuller answer below.
Here’s the thing with this (and when I’ve been asked similar questions about other languages). There’s no word in Dothraki that MEANS “princess”, if you follow. The Dothraki don’t have kings and queens. If you don’t have kings and queens, you don’t have princes and princesses. The Dothraki also haven’t gotten to a point where they’re regularly discussing the political systems of other cultures in detail, so they haven’t borrowed or invented terms for specific political relationships that aren’t a part of their own political system. For example, consider the term “the hand of the king”. That’s a very specific role in the government of Westeros. You could translate that literally into Dothraki, but it would be meaningless (or rather it would translate as “the khal’s hand”, and it wouldn’t be obvious from the construction that it refers to some political appointment). You could also explain very easily in Dothraki the function of the hand of the king and everyone would understand. You could also say ko. But it would be a big mistake to translate ko as “hand of the king”. That would make no sense. Not only are the two things not identical, but the hand of the king isn’t something that’s regularly discussed in Dothraki. No one you spoke with would have any preexisting knowledge about the role.
So back to “princess”, there are terms for the son and daughter of a khal, because there was one for “son” in the books: khalakka. It’s translated as “a prince rides in side me” (khalakka dothrae mr’anha), but I don’t think you can it means “prince”. It’s not compatible with our understanding of what a prince is—it’s just close. So just as a khal is not a king, a khalakki is not a princess. It’s just close.