This definitely reminded me of some my old favourites from my childhood. Books like The Land of Green Ginger, or The Wind on the Moon, and anything by E Nesbit where the authors didn't talk down to her/his readers and weren't afraid of using complicated words and phrases because they gave a richness and lyricism to the story. Many other authors would avoid this and choose something more simplistic instead.
And September is a lovely heroine. Not perfect by any means. She makes mistakes and can be a bit irascible. But her heart is in the right place and she's loyal and loving to her friends.
If I'd read this aged ten, I would have wanted a wyvern of my very own.