There's a workshop I give in Primary Schools where I ask children to draw a place they know really well - where they can look around and know exactly what is there. It could be their bedroom, their garden, a playground. Once I have them in this familiar place, they have to think of something that could be the entry point to an imaginary world. This task always grabs their imagination; even the most unimaginative pupils come up with a point of entry: a shaft of light from a window falling on a certain floor board in their bedroom; a particular wood pattern on the garden fence that looks like a door; a swing in the playground that, when it reaches a certain height, catapaults them into another world. The key point is to establish a very real and ordinary world before attempting to enter any imaginary world.
The illustrators here draw two such very ordinary places - Arthur Rackham, a rabbit hole in Alice in Wonderland and Pauline Baynes, a wardrobe in The Lion the Witch and the Wardrobe. Neither of them show the entry actually taking place; that is left for the text to describe and the reader to imagine. But in a picture book, more is shown in the illustrations. I talk about this in the previous post GOING BETWEEN WORLDS where I illustrate the scary moment of entry into the world inside a tree in The Little White Sprite.