My two boys love having a 3D printer in the house. I have made a variety of items for them. My 9 year old, Derek, came up with a great little Lego build. He made a tiny Lego version of the Printrbot simple metal 3D printer that we have.
He really captured the essence of the simple metal with incredibly few Lego pieces. It is even sized about right to put it in the workshop in their Lego city.
The parts you will need to build your own and an instructional video by Derek are below.