Give your child a tape measure and measure the room together; depending on your child's age, this may be an opportunity to work on adding feet and inches. Your child can apply those dimensions to visual images of the room.
Take a photograph of your child's room (empty, if possible). Enlarge it on a photocopier for a more workable scale (make several copies). Or, computer enthusiasts might enjoy scanning in the photo to make their own printouts and enlargements.
If you don't have a photo of the empty room, use measurements and basic line drawings to replicate the room, including windows and doors. Graph paper will make it easier to get the dimensions right. You may want to make some photocopies of this drawing, so your child can experiment with color at the next stage.
Have your child use the photo to pick out carpet and wall colors and to sketch window treatment designs, using other articles on this site or in magazines for inspiration. He or she can try out ideas and color preferences, and show you on the photocopies using colored pencils.
To help children understand spatial relationships, take measurements of the furnishings for the room. If you haven't already done so, outline the room's dimensions on a piece of graph paper, and indicate window and door placement. Note the scale of the graph paper (typically, 1/4 inch equals 1 foot). Use the same scale to draw and cut out corresponding furniture shapes from colored paper. Label them and let your child arrange the pieces until he or she finds a pleasing placement. Point out potential problems such as not being able to open dresser drawers or blocking traffic flow.