If you want to introduce a kid (or yourself!) to CAD (computer aided design), Tinkercad is by far the easiest and most fun way to begin. Today I mentioned to my 10-year-old that our CNC machine would soon be up and running. He asked what a CNC could do, and I said one example would be to carve a battlefield out of stiff foam for Warhammer figures.
That got his attention 
. He wanted to know how to tell the CNC what to do. I explained a bit about CAD, and showed him Tinkercad, giving the example of one cube that you could stretch and change.
Then I got busy with something else and left him to figure out Tinkercad himself. I came back an hour later and the below is what he’d designed. A ten-year-old. No training. One hour.
The green stuff we’re going to CNC out of a sheet of stiff foam. The rest we’ll probably 3D print on the Makerbot. It will take a weekend, but this could be our first 100% digital craft project.
This is an example of what I talk about in Makers: manufacturing technologies are getting so easy and cheap (even free) that anyone can use them. Kids today can grow up as fluent in CAD as they are in everything else on computers. Democratizing the tools of publishing brought us the Web. Just imagine what democratizing the tools of manufacturing will do.
We’ve used the previously reviewed Sketchup and Autodesk 123D, and both are great. But Tinkercad just runs in your Web browser and its simple interface disguises a very sophisticated cloud-based CAD engine.
— Chris Anderson
Tinkercad
Free trial, or $19/month for personal account (pro accounts vary)