Blender is free, open-source software that can be used to create animations and 3d art (and can also be used to create designs for 3d printing).
A 3-minute animated short, Glass Half, was created with Blender:
Here is another one, Katsu Kats.
Here is another short film, created by the Blender Institute (note that the dragon “realism” may be slightly disturbing for younger viewers):
There are free Blender tutorials, for absolute beginners including this one: