There are short MG books out there, though most of them were published in the pre-Potter days. SARAH, PLAIN AND TALL (8K) and THE WHIPPING BOY (13K) are good examples. And they won Newbery awards, so I don't think anyone would argue that they'd be better books if they were longer.
That said...the standard these days is to be longer. 25-30K. And if you read the Whipping Boy and compare it to a more contemporary fantasy novel, you can really see the difference. A longer book gives you more time to develop setting and character. It gives you a more satisfying reading experience.
Think of it this way: If I fall in love with the world and characters you've created, I want to stay with them for as long as possible. You know that bittersweet feeling you get when a good book ends?
Without reading your book, I have no way of knowing if you have simple perfection like Sarah, Plain and Tall, or if you have the bones of a story that would be more satisfying if you fleshed it out more. But I do know that your chances of getting it published are much better if you double the word count. And I don't mean adding random things to make it longer. I mean finding ways to make your story richer and more complex.
If that feels like the wrong thing to do, then Lmc is probably right. This might be something you want to pursue publishing in a less traditional way, like ebooks or small press.