Very good question. If Sarah stays the size she is, she will be the size of a typical 10 year old girl for the rest of her life. Does this qualify her as a 'little person'?
I just looked it up and a 'little person' is no taller than 4'10". Interesting. Sarah is 4'8".
But, I digress. The issue is, what contributes to Sarah's petite size? She is a very good eater. Very healthy. No medical issues. I find most people with cerebral palsy to be larger than Sarah in adulthood. Of course, I speak in the dangerous world of generalities here. Shall I just say, I don't think that CP is the answer in Sarah's situation.
I am inclined to say that whatever Sarah 'has' is responsible for her small size. It is part of the 'undiagnosed diagnosis', being one of its many unusual features. I am aware that girls with Rett syndrome tend to be on the small size.
One day soon, I will do a post showing which traits Sarah has that fit the Rett category and those that don't. That would be kind of interesting.
So John, there's your answer. In a round-about way.