Building structures out of sun dried earth is one of the oldest construction methods there are, and with this fun project kids can try their hand at early architectural techniques and build a miniature playhouse in the garden while they're at it!
Adobe, mudbrick and cob structures all use similar ingredients (sand, clay, water and some sort of binder like straw or sticks), and can be easily made in miniature version in the garden. After mixing up our batch of mud (we didn't have straw so we substituted some grass clippings and the fibrous parts of maple seeds) we pressed it into an ice tray with removable dividers. If you don't have one of these, the flexible versions would work well too, but a traditional version wouldn't be the best choice because the bricks would be hard to remove without falling apart.
Making strong bricks requires a fair amount of clay in the mixture. One way you can tell if your dirt will be good for bricks is to mix it with water in a clear jar until it is almost completely saturated and then shake it up... kids love this part. After the mixture is shaken for a while (about a minute or so), and then placed on a table to allow it to settle you will be able to tell how much clay there is because it will fall to the bottom of the jar.
... you could add a few embellishments to invite the elves or fairies to find their way in.