This is my take on a child door knob safety cover. I needed something to cover 20+ doors and didn't want to spend $60+ on them. I also didn't want to deal with extra hardware like screws or nuts to keep the things together, so this simply snaps together over the knob.
(Fair warning, this is fairly difficult to remove from the door if you use tight allowances around the knob.)
This print, as is, takes roughly 15-20g of filament, so $.35-$.40 with a $20 roll roughly. It's designed to fit a round door knob with a 51mm diameter. The files for "no lock" cover the entire knob with the exception of the spaces for your thumb and forefinger to grip the knob. The other files allow access to the lock.
BTW, glow in the dark knobs are pretty sweet when your room is pitch black at night. With lower infill percentages, the glow is low enough not to disturb sleeping little ones!
The OpenSCAD file for this can be found here and allows for adjusting the front removal, rear removal and overall size of the knob as well as several allowances.
Female Side: https://www.thingiverse.com/thing:3347343
Male Side: https://www.thingiverse.com/thing:3347342
I used inclusive for the "Slicing Tolerance" in Cura to ensure that the snap fittings fit properly. I also tested and had decent luck with variable layer heights, as long as the snap fitting themselves were printed close to .1mm layer heights.