No assembly necessary. Just print and play.
The bores on the gears should be a snug fit on a 1cm shaft, but not so snug that a child can't easily attach and remove them.
A piece of sandpaper or masking tape will help adjust the bore and dovetails to the desired tightness.
The gears have 32, 16, and 8 teeth, respectively. Computing gear ratios is left as an exercise for the reader.