Right now 3D printing has limits that prevent kids from downloading and printing a functional gun, the risk right now is that they'd hurt themselves trying.
Anyone who can make a 3D printed gun work at the moment could also use other tools to make one out of a pipe and other parts found at a hardware store.

I agree it's a scary vision, but the neat apsects of 3d printing prevails.
New products, medical applications, art....

As with other technology, educating kids about how to use it right, is the best way to pretend awefull things to happen.
Bans are usualy just making things worse.