How do you write children as children?

Patrick Gale knows and you can see it in his Notes of an Exhibition. He has the siblings Garfield and Morwenna visit their mother in mental hospital where she has given birth to the new baby Hedley. The way they are shown to react has such an authentic feel to it. We are told when writing for children to write to their height. Gale knows exactly what that height is. We can say that his book is not strongly plot driven but the characters and their interaction is utterly compelling.