I've been using Tinkercad for a while but hadn't mentioned it before because it has been discontinued due to the developers having other priorities. But now we have some excellent news, Tinkercad have been bought by Autodesk, making of professional 3D modeling tools. You can now create a new account with Tinkercad and Autodesk plan to continue to develop the service, yay!
There's a free version which has been upgraded since the Autodesk acquistion and has lots of features for getting started. Tinkercad works by dragging and dropping 3D shapes and is very intuitive for beginners, especially if they have worked with a documentation or presentation editor. You can also use shapes to make holes in objects which is very useful.
I'm very pleased to see Tinkercad back, we'll certainly be using it again now I know it has a future. Watch the video below to get an overview of the capabilities and then get your kids to give it a try and design some cool 3D objects.
Update: We've added a tutorial for creating a children's name keychain using Tinkercad.