If there is one program that anyone needs to have who is interested in 3D printing (or knows of a child that may be interested) — it is Tinkerplay. It runs on Windows, Android, Windows phone and iOS.
You can build various ‘creatures’ out of predefined parts. You can play with them in the building environment:
That’s an animated gif of a number of screenshots in a rendered environment.
Once you have everything defined the way you want, you can then build a parts list model that is ready to print: