Comment by badlogic

I used PLA, which is a non toxic bio plastic. ABS is also an option for at-home 3D printing, which is the material used in lego bricks. At his age, he doesn't put anything in his mouth anymore, so swallow hazards were not a concern. That said, the only thing that's small enough for him to swallow are the buttons and the knob, which can not be detached from the device without unscrewing the enclosure. If he is able to do that, nothing is save.