TinkerCAD was very popular with 3D printing people, because it was stupidly simple and easy to learn, while still being a very functional CAD tool.
Basically Tinkercad had two unique advantages: a world-class massively parallel solid geometry kernel, and a 3D design UI that made ease of use first priority. Tinkercad emphasized the latter in their product image, but it became somewhat pigeonholed as a "children's CAD", which made it difficult to switch the same product into a pro gear. They've now made the decision to pursue the revenue potential of the pro market, and that requires a new product image.
If someone wants to jump onto the opportunity of building a Tinkercad replacement, I know a few things about this space and am available for hire... ;)
It's going through a major transition at the moment (http://shapesmith.net/2013/03/06/ProgressUpdate.html), and not necessarily kid-friendly, but it's open source so someone could create a kid-friendly interface