If you have a child that enjoys fairies and elves and unicorns and such, then this is something else for them to watch. It's a fantasy show where a real life (live action) teenage girl is magically transported to a (CG animated) land of flying elves on occasion.
Season 1 is formulaic with the new girl in school and the school bullies and the outcast friends. Lessons are learned, friends are made, bullies get their comeuppance, and in the fantasy land the heroes band together to defeat the villain.
Season 2 gets more complex. The focus is on the main character and her school bully as they both spend a summer on their respective families' houses in the country. The stakes are higher and we learn more about the families of the characters. The antagonist even gets to join in on the fantasy adventures and gets her own redemption arc. The fantasy story involves freeing animals and finding characters and solving puzzles and has a little more to it than the last season.
Season 3 is weird and different but ultimately inoffensive. All of the actors are replaced, there's a new Mia and a new voice cast and new characters. The fantasy story is just a rehash of Season 1. The new cast is fine. It's admittedly less engaging than the first two seasons, but it'll do for anyone wanting to see Mia go on more adventures.
It's fine.