Through the window of the book—that is to say, Stevenson's collection of poems for children A Child's Garden of Verses—it is possible for the reader to see a different world, a faraway land in which a child is playing in his garden.
However, the child in the book is not like the reader. For the reader, whom we must presume to be a child, is watched over by his mother whenever he goes out to play in the garden. She keeps a close eye on him through the window, and if she ever needs to call her son, she can simply knock on the window.
But such a luxury is not available to the reader. He or she cannot call the boy in the book away from his play, as he is nothing more than an idea; he isn't real; he has no existence outside the text.
The boy in the book cannot hear the reader. Nor can he see him or in any way be lured by him from the confines of the book's pages. As such, he can happily continue to concentrate on his games, merrily playing away:
Is all on his play-business bent.
Nothing that we as readers can do or say will ever change this fact. All we can do is observe.