Some hospitals, including Arkansas Children's Hospital in Little Rock, have special programs to help children understand where they're going and what will happen while they're there. There's also at least one book -- Going To The Hospital, by Fred (Mister) Rogers, aimed at preparing kids for a hospital stay.
These programs explain things like the bracelet they'll get in the admissions office, about who will help them get settled in their room, and what they'll wear there, what they'll eat, and where they may be able to play.
Other possible questions include: Will they meet other kids? Will they meet lots of doctors? Will they have to get a shot? Where? Will it hurt? Will they have to take pills?
At the Arkansas Children's Hospital orientation session for kids and their parents, an audiovisual presentation shows them what happens when a child is hospitalized. Then there's a hospital tour, during which the child learn about hospital equipment. Opportunities to ask questions also help ease the child's mind, as do pamphlets that can be taken home.
Fred Rogers' book uses photographs to support his typically straight-forward approach to his subject.
However you approach it, your effort to prepare your child for the hospital should be age-appropriate, and very much focused on what the child wants to know.