PLEASE BE CAREFUL WITH THIS KNIFE. It is NOT a toy. 

EDIT:  I HAVE UPDATED THE FILES.  I fixed a safety concern for the design of the knife.  I moved the button to the opposite side, so there is no way of slicing your finger while pulling the blade in and out.  Because of some internal geometry, you will need to print the handle in this position with supports.

I printed mine out of PETG-HF.  PLA would probably be okay as well, but the spring button might be a little stiffer and harder to use.

This video shows the old model.  The new version (updated in the STL and Print profile) has the button on the opposite side of the handle.