Get ready, Shmoopers, because he have a lot to say here. Let's start with the easy:
A Little Princess could mean two different things: it could be an acknowledgment of Sara's good breeding, or it could be a straight-up insult. When Sara has her money and wealth, folks at the school like Miss Minchin use this nickname in order to compliment her behavior. Others, though, like Lavinia use it as a sarcastic jab—as if Sara is sooo very full of herself that she acts like she's royalty when she's not.
And then once she descends into poverty though, the nickname becomes somewhat of a cruel joke. A joke in the "let's laugh at the poor girl and call her a princess because she's so dirt poor!" (Yeah, we think they're jerks too.) Sara definitely has the last laugh though, because in the end she rises above her pauper status and regains her wealth. Who's the princess now, Lavinia?!
Ready to kick things up a notch? Consider this: A Little Princess was a stage play before it was a novel (although after it was a story with a totally different title). And as a play, it was called The Little Princess.
Big deal, Shmoop. "A" instead of "The." So what?
Well, we do think it's a big deal. If the story is called The Little Princess, then it's specifically about Sara. Sara is the little princess, because she has inner nobility that's unique and special to her.
But if the story is called A Little Princess, then Sara is just one of many princesses. She's an example of how every little girl could be. Sure, she's special—but we could all be special, if we followed her example. We could all be little princesses.
See? Pretty big difference.