TL;DR: the result is here.
My 6 years old son made a lego experiment. The piece in paper is supposed to be rotated fast so that we can see the motorcycle inside the flame wheel.
Yet, the piece being in paper, it broke easily.
Let’s try to print a part in which he could glue the disks.
The axis should fit in a square of 4.5mm X 4.5mm.
Actually, the square axis appears to be bigger than that, so I may want to try 5mm for a start. It is smaller than the diagonal of the axis (4.3 * sqrt(2) = 6.08) so it should be ok.
The square axis should be about 1.9 cm long.
And the disks should have a diameter of ~5.9 cm
For the axis to get into the piece, I need to make a hole into the disk supports. The disks supports may actually be a cylinder of length 5mm.
Result
With a few minutes of blender, creating a cylinder, two cube, a few resizing and a few boolean operations, this was an easy one.