Il y a un ordinateur sur le bureau
Il y a des livres sur les étagères.
Il y a une affiche sur le mur.
Some teachers might find this approach a little predictable, but it serves a valid linguistic purpose and relates to the child's own experience.
Once the key language has been acquired (vocabulary, prepositions and il y a) you could spice up the task by getting students to work in pairs describing a simple aerial view of a bedroom (or any room, for that matter). To help them you would need to provide them with other phrases such as: dans le coin, à gauche, à droite, au milieu. Pupil A describes their picture to pupil B who has to draw it on a blank piece of paper or mini whiteboard. Then pupil B describes their picture to pupil A. Good pairs may well turn this into a dialogue with some limited questioning: la chaise, c'est à gauche de la porte ou à droite?
Weaker groups could be given plenty of written support on a help sheet or on the board.
This would be a good 20 minute task towards the end of a teaching sequence on prepositions or could easily be used with a more advanced group for revision.
Pupils could design their own simple overhead views of rooms or you could find them online. The one above came up in a Google search.
There is also James Mollison's well publicised Kids' Bedrooms Around the World which could be used. It is mentioned here, for example. It has stronger cultural value, but may make less sense to the teacher as overhead views are easier to describe and the Mollison pictures contain less controllable vocabulary. I would be tempted to use these almost as an afterthought, not as part of the main teaching sequence.