The main characters in fiction for children and teenagers tend, not surprisingly, to be children and teenagers, though it's not hard to find exceptions, such as Philip Pullman's Once Upon a Time in the North. To write convincingly, whether in first- or third-person, you need to position yourself inside the head of one or more characters. In Tom's Midnight Garden, we share Tom's thoughts all the way: his frustration at being cooped up, his interest in the old grandfather clock, his surprise at finding that the midnight garden is different from the daytime one.
One way of getting a sense of your characters as rounded human beings, rather than as cardboard cut-outs, is to build them through questions and answers. For example: What's in her pocket? Who does she dislike, and why? What's her best subject at school? Who would she most like to get a text message from? What's she most anxious about? and so on. And it's important to hear your character speaking, and to see his or her body language.
Adults will almost inevitably appear, but children's writers are adept at getting rid of them, or at least keeping them on the sidelines, so that the children have to confront their own difficulties. Health and safety consciousness can curtail the activities of children in present-day stories of the real world, which may account for the huge amount of fantasy published in recent years; in imaginary settings, child characters can be magicians, warriors, seers, time-travellers, or whatever the author wants them to be. Similarly, children in historical fiction can plausibly face huge responsibilities and go on dangerous journeys with only their own resources to depend on.
