By J.K. Rowling
The one where a boy who has been sleeping in a cupboard under the stairs finds out he is a wizard, and your child finds out that a big fat book can swallow them whole.
Harry Potter is an orphan who lives under the stairs at his awful aunt and uncle's house, until letters start arriving that his relatives try desperately to hide. On his eleventh birthday a giant of a man named Hagrid finds him and tells him the truth: Harry is a wizard, famous in a world he has never seen, and there is a place for him at Hogwarts School of Witchcraft and Wizardry. He makes his first real friends in Ron and Hermione, learns to fly a broomstick, and stumbles onto a secret hidden in the school, a stone that can make its owner live forever. Someone wants it badly, and that someone is tied to the dark wizard who murdered Harry's parents when he was a baby.
This is the classic first doorstopper, the book more children have crossed over on than any other. The chapters are a comfortable length and the story moves, so the page count never sits heavy on a reader who is used to shorter books. The pull is the world itself, a school your child will want to walk around in, and the deep wish underneath it, that a child nobody wanted might turn out to matter enormously. Fall for this one and there are six more waiting on the shelf, each thicker than the last, so it is the gentlest possible on-ramp to a lifetime of long books.