MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
My son was using Tinkercad since he was 6yrs old and I would print his creations. You could also make clay or playdough models with them and scan them using the digitizer or other less expensive 3d Scanners. We use a Kinect Scanner with Skanect software and it works pretty well and is very inexpensive. Scanning is hard for young hands though. I am looking at the atlas scanner http://www.filastruder.com/collections/atlas-3d as an alternative. If they use minecraft you can also build "things" in minecraft and bring them into Tinkercad. It is actually pretty easy and the kids can print their minecraft creations. I have known preschoolers to be proficient at minecraft.