Your child hasn't read fantasy yet, or picked one up once and put it straight back down. This is the shelf to start from. Six books that hand over dragons and magic and secret worlds without asking anyone to carry a five-hundred-page epic first. There's no wrong pick. Choose by your child's age, and by which flavor of fantasy tempts them: funny, a quest, an animal at the heart of it, or magic hiding inside our ordinary world. Then hand it over.
The reason a first fantasy so often fails is the front of the book. A made-up world, a map, forty names to learn before anything happens, and a child decides fantasy isn't for them. Every book here solves that a different way, and any one of them will do the job on its own. So steer by taste. If your child wants funny, hand them The Creature of the Pines, which opens on an ordinary school field trip in New Jersey and lets a monster walk in thirty pages later. If they want an animal at the center, Zoey and Sassafras is a girl and a cat looking after magical creatures with a science notebook, and it's the softest thing on the shelf. If they want a hero their own size with a power, Dragon Masters gives them a boy, a dragon, and short chapters with a picture every few pages. If school-shaped stories are more their thing than quests, Upside-Down Magic never leaves the classroom, and the whole plot is magic that won't behave. Then there are the two bigger ones, and they're bigger in feeling as well as length. The Tale of Despereaux is a beautiful, dark little fairy tale about a mouse who loves a princess. The Land of Stories drops twins into a world where every fairy tale is real and runs for four hundred and forty-eight pages. Those two are for a child who's ready. Nobody has to work up to them, and a child who reads only Zoey and Sassafras and stops there has still found fantasy.