There are many ways to make a den but by far the quickest and easiest is to drape a sheet over a washing line. Pull the two sides out and weigh down the corners with books or large stones, so that it keeps its shape.
For a more spacious construction, try running two washing lines in parallel, a few feet apart and hanging the sheet over both. On a still day, with this type of tent you won’t even need to secure the corners as the sides will hang straight down.
Add rugs, blankets and cushions for extra comfort and don’t forget stock up on books, toys and rations!